Changing empty placeholder on MaterialListValueBox does not work correctly

[chemmett]

If you call setEmptyPlaceHolder() on a MaterialListValueBox that has an existing placeholder, it inserts another placeholder at index 0 instead of just changing the text. If you change it when the list is empty, you end up with multiple placeholders, if you change it when there are already values in the list, the side effects are worse: it changes the indexing and the selected value does not equal the visually selected item.

Our use case was we wanted the placeholder to say something like "loading..." initially and then change it to "Select Foo" when the list populated after an ajax call.

Instead of always calling insertEmptyPlaceHolder(value), it should check if the placeholder is listed and if so, just change the text.
emptyplaceholder.txt