[Lens] Fix buggy handling of user pressing delete on the field selector

[wylieconlon]

I expect that if you press delete on a field, that you would get an error message. But instead, we get an empty label:

These are the steps needed to test this:

1. Select Average of bytes or any other field
2. Open the field selector, and press the Delete key
3. Close the dimension without finishing your work

The correct behavior is already shown when dealing with reference-based operations like Moving Average. Steps to see the correct behavior:

1. Drag Records into your workspace to get a date histogram chart
2. Select the Y axis, and switch to Moving Average
3. Open the field selector and press delete
4. Error state is shown

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)