Use 'start' as default instead of undefined for align attribute in dialog actions

Author: PooSham

src/material-experimental/mdc-dialog/dialog-content-directives.ts

@@ -148,7 +148,7 @@ export class MatDialogContent {}
   },
 })
 export class MatDialogActions {
-  @Input() align?: 'center' | 'end' = undefined;
+  @Input() align?: 'start' | 'center' | 'end' = 'start';
 }
 
 /**

src/material/dialog/dialog-content-directives.ts

@@ -152,7 +152,7 @@ export class MatDialogContent {}
   },
 })
 export class MatDialogActions {
-  @Input() align?: 'center' | 'end' = undefined;
+  @Input() align?: 'start' | 'center' | 'end' = 'start';
 }
 
 /**