[gdb/testsuite] Fix xfail handling in gdb.threads/gcore-thread.exp

[gdb/testsuite] Fix xfail handling in gdb.threads/gcore-thread.exp

When running test-case gdb.threads/gcore-thread.exp on openSUSE Tumbleweed,
I run into these XFAILs:
...
XFAIL: gdb.threads/gcore-thread.exp: clear __stack_user.next
XFAIL: gdb.threads/gcore-thread.exp: clear stack_used.next
...

Apart from the xfail, the test-case also sets core0file to "":
...
        -re "No symbol \"${symbol}\" in current context\\.\r\n$gdb_prompt $" {
            xfail $test
            # Do not do the verification.
            set core0file ""
        }
...

After which we run into this FAIL, because gdb_core_cmd fails to load a
core file called "":
...
(gdb) core ^M
No core file now.^M
(gdb) FAIL: gdb.threads/gcore-thread.exp: core0file: \
  re-load generated corefile
...

Fix this FAIL by skipping gdb_core_cmd if the core file is "".

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-04-06  Tom de Vries  <tdevries@suse.de>

	PR testsuite/27691
	* gdb.threads/gcore-thread.exp: Don't call gdb_core_cmd with core
	file "".