Fix bug that could cause map_fn to produce incorrect results (rather than an error)
when mapping over a ragged tensor with an inappropriate fn_output_signature.  (Note: there are cases where the default value for fn_output_signature is not appropriate, so the user needs to explicitly specify the correct output signature.)

PiperOrigin-RevId: 387606546
Change-Id: Ib4ea27b9634e6ab413f211cfe809a69a90f0e2cd