[editor] Fix style property inputs when value is set to empty string

The whole panel tab was not correctly checking for value existence. Need 
to check for undefined not null.

projects/editor/src/app/unit-view/page-view/properties-panel/element-style-properties.component.ts

@@ -7,31 +7,31 @@ import { UIElement } from '../../../../../../common/models/uI-element';
   selector: 'app-element-style-properties',
   template: `
     <div fxLayout="column">
-      <mat-form-field *ngIf="combinedProperties.backgroundColor"
+      <mat-form-field *ngIf="combinedProperties.backgroundColor !== undefined"
                       appearance="fill" class="mdInput textsingleline">
         <mat-label>{{'propertiesPanel.backgroundColor' | translate }}</mat-label>
         <input matInput type="color" [value]="combinedProperties.backgroundColor"
                (input)="updateModel.emit({ property: 'backgroundColor', value: $any($event.target).value })">
       </mat-form-field>
-      <mat-form-field *ngIf="combinedProperties.backgroundColor"
+      <mat-form-field *ngIf="combinedProperties.backgroundColor !== undefined"
                       appearance="fill" class="mdInput textsingleline">
         <mat-label>{{'propertiesPanel.backgroundColor' | translate }}</mat-label>
         <input matInput type="text" [value]="combinedProperties.backgroundColor"
                (input)="updateModel.emit({ property: 'backgroundColor', value: $any($event.target).value })">
       </mat-form-field>
-      <mat-form-field *ngIf="combinedProperties.fontColor"
+      <mat-form-field *ngIf="combinedProperties.fontColor !== undefined"
                       appearance="fill" class="mdInput textsingleline">
         <mat-label>{{'propertiesPanel.fontColor' | translate }}</mat-label>
         <input matInput type="color" [value]="combinedProperties.fontColor"
                (input)="updateModel.emit({ property: 'fontColor', value: $any($event.target).value })">
       </mat-form-field>
-      <mat-form-field *ngIf="combinedProperties.font != null"
+      <mat-form-field *ngIf="combinedProperties.font !== undefined"
                       appearance="fill" class="mdInput textsingleline">
         <mat-label>{{'propertiesPanel.font' | translate }}</mat-label>
         <input matInput type="text" [value]="combinedProperties.font"
                (input)="updateModel.emit({ property: 'font', value: $any($event.target).value })">
       </mat-form-field>
-      <mat-form-field *ngIf="combinedProperties.fontSize != null"
+      <mat-form-field *ngIf="combinedProperties.fontSize !== undefined"
                       appearance="fill" class="mdInput textsingleline">
         <mat-label>{{'propertiesPanel.fontSize' | translate }}</mat-label>
         <input matInput type="number" #fontSize="ngModel" min="0"
@@ -40,7 +40,7 @@ import { UIElement } from '../../../../../../common/models/uI-element';
                                                    value: $event,
                                                    isInputValid: fontSize.valid && $event !== null})">
       </mat-form-field>
-      <mat-form-field *ngIf="combinedProperties.lineHeight != null"
+      <mat-form-field *ngIf="combinedProperties.lineHeight !== undefined"
                       appearance="fill" class="mdInput textsingleline">
         <mat-label>{{'propertiesPanel.lineHeight' | translate }}</mat-label>
         <input matInput type="number" #lineHeight="ngModel" min="0"
@@ -50,17 +50,17 @@ import { UIElement } from '../../../../../../common/models/uI-element';
                                                    isInputValid: lineHeight.valid && $event !== null })">
       </mat-form-field>
 
-      <mat-checkbox *ngIf="combinedProperties.bold != null"
+      <mat-checkbox *ngIf="combinedProperties.bold !== undefined"
                     [checked]="$any(combinedProperties.bold)"
                     (change)="updateModel.emit({ property: 'bold', value: $event.checked })">
         {{'propertiesPanel.bold' | translate }}
       </mat-checkbox>
-      <mat-checkbox *ngIf="combinedProperties.italic != null"
+      <mat-checkbox *ngIf="combinedProperties.italic !== undefined"
                     [checked]="$any(combinedProperties.italic)"
                     (change)="updateModel.emit({ property: 'italic', value: $event.checked })">
         {{'propertiesPanel.italic' | translate }}
       </mat-checkbox>
-      <mat-checkbox *ngIf="combinedProperties.underline != null"
+      <mat-checkbox *ngIf="combinedProperties.underline !== undefined"
                     [checked]="$any(combinedProperties.underline)"
                     (change)="updateModel.emit({ property: 'underline', value: $event.checked })">
         {{'propertiesPanel.underline' | translate }}