[gdb/cli] Don't assert on empty string for core-file

[gdb/cli] Don't assert on empty string for core-file

With current gdb we run into:
...
$ gdb -batch '' ''
: No such file or directory.
pathstuff.cc:132: internal-error: \
  gdb::unique_xmalloc_ptr<char> gdb_abspath(const char*): \
  Assertion `path != NULL && path[0] != '\0'' failed.
...

Fix this by skipping the call to gdb_abspath in core_target_open in the
empty-string case, such that we have instead:
...
$ gdb -batch '' ''
: No such file or directory.
: No such file or directory.
$
...

Tested on x86_64-linux.

gdb/ChangeLog:

2021-08-30  Tom de Vries  <tdevries@suse.de>

	PR cli/28290
	* gdb/corelow.c (core_target_open): Skip call to gdb_abspath in the
	empty-string case.

gdb/testsuite/ChangeLog:

2021-08-30  Tom de Vries  <tdevries@suse.de>

	PR cli/28290
	* gdb.base/batch-exit-status.exp: Add gdb '' and gdb '' '' tests.