[Ada] fix warning when printing empty array

[Ada] fix warning when printing empty array

This patch should fix the following regression:

     (gdb) print my_table
    -$1 = ()
    -(gdb) PASS: gdb.ada/null_array.exp: print my_table
    +$1 = (warning: unable to get bounds of array, assuming null array
    +)
    +(gdb) FAIL: gdb.ada/null_array.exp: print my_table

The problem was introduced by a change in val_print_array_elements
which removed a check for the case where the array's high bound
is smaller than the array's low bound (empty array).

This change restores the check and forces the len to zero in that case.
Looking at the patch that caused the regression, I suspect that we may
have other parts that might have been broken (non-zero array low bound?).

gdb/ChangeLog:

        * valprint.c (val_print_array_elements): Put back handling of
        empty arrays.