message: fix(DateRange): set default placeholder value
  The null value was overriding the default set by the DatePicker
diff: @@ -85,7 +85,7 @@ export class TsDateRangeComponent {
  * Define the start date placeholder
  */
  @Input()
  - public startPlaceholder: string;
  + public startPlaceholder: string = 'Select a date';
  /**
  * Define the max date for the starting date
  @@ -109,7 +109,7 @@ export class TsDateRangeComponent {
  * Define the end date placeholder
  */
  @Input()
  - public endPlaceholder: string;
  + public endPlaceholder: string = 'Select a date';
  /**
  * Define the max date for the end date
  