Remove some null checks

Remove some null checks

When not parsing for completion, parse_expression ensures that the
resulting expression has operations.  This patch removes a couple of
unnecessary checks for this situation.

gdb/ChangeLog
2021-03-08  Tom Tromey  <tom@tromey.com>

	* printcmd.c (set_command): Remove null check.
	* value.c (init_if_undefined_command): Remove null check.