[flang] Handle empty array references in DATA statements

When an array reference in a DATA statement is empty due to an
empty vector subscript or a section lower bound being higher than
its upper bound with a positive stride (or lower with negative),
ensure that isEmpty() is correct afterwards so that such an empty
array reference doesn't terminate processing of that DATA statement
block.

Fixes https://github.com/llvm/llvm-project/issues/63512.

Differential Revision: https://reviews.llvm.org/D153799