fix(editors): add blank entry on Select Editor should happen once (#591)

- the problem was noticeable after opening/closing the select dropdown multiple times

src/app/modules/angular-slickgrid/editors/selectEditor.ts

@@ -519,7 +519,7 @@ export class SelectEditor implements Editor {
     }
 
     // user can optionally add a blank entry at the beginning of the collection
-    if (this.collectionOptions && this.collectionOptions.addBlankEntry) {
+    if (this.collectionOptions && this.collectionOptions.addBlankEntry && Array.isArray(collection) && collection.length > 0 && collection[0][this.labelName] !== '') {
       collection.unshift(this.createBlankEntry());
     }