[flang] Fix edge-case I/O regressions

A blank field in an input record that exists must be interpreted
as a zero value for numeric input editing, but advancing to a
next record that doesn't exist should leave an input variable
unmodified (and signal END=).  On internal output, blank fill
the "current record" array element even if nothing has been
written to it if it is the only record.

Differential Revision: https://reviews.llvm.org/D118720