[flang][runtime] Handle empty NAMELIST value list

[klausler]

InputNamelist() returns early if any value list read in by InputDerivedType() or DescriptorIo() is empty, since they return false. But an empty value list is okay, and the early return should occur only on error.

Fixes #151756.

[eugeneepshteyn]

Unit test?

[akuhlens]

How does checking if the handler has signalled an error correspond to the empty list? It isn't obvious where there error is coming from, though I assume if I stared at the code long enough I would be able to figure it out.

[klausler]

How does checking if the handler has signalled an error correspond to the empty list? It isn't obvious where there error is coming from, though I assume if I stared at the code long enough I would be able to figure it out.

The original code was assuming that there must have been an error if no items had been read. But sometimes there are just no items in the input to be read, and this is not an error.

[akuhlens]

LGTM

[DanielCChen]

LGTM.
Thanks!