Avoid self-move-assign of all elements in vector erasing empty

[Billy O'Neal (BillyONeal)]

Avoid self-move-assign of all elements in vector when erasing an empty range.

Resolves DevCom-776568.

Description

This guards the _Move_backward attempt such that some movement actually occurs.

Checklist

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[Casey Carter (CaseyCarter)]

Avoid self-move-assign of all elements in vector erasing an empty range.

This is hard to parse: maybe "when erasing"?