remote: Return NULL extra_info/name if they are empty

remote: Return NULL extra_info/name if they are empty

Commit

  remote: C++ify thread_item and threads_listing_context
  21fe1c752e254167d953fa8c846280f63a3a5290

broke the test gdb.threads/names.exp.  The problem is that since we now
use an std::string to hold the extra_info, an empty string is returned
by target_extra_thread_info to print_thread_info_1 when the remote stub
didn't send any extra info, instead of NULL before.  Because of that,
print_thread_info_1 prints the extra info between parentheses, which
results in some spurious empty parentheses.

  Expected: * 1    Thread 22752.22752 "main" all_threads_ready () at ...
  Actual  : * 1    Thread 22752.22752 "main" () all_threads_ready () a ...

Since the bug was introduced by a behavior change in the remote target,
I chose to fix it on the remote target side by making it return NULL
when the extra string is empty.  This will avoid possibly changing the
behavior of the common code and affecting other targets.

The name field has the same problem.  If a remote stub returns no thread
names, remote_thread_name will return an empty string instead of NULL,
so print_thread_info_1 will show empty quotes ("") instead of nothing.

gdb/ChangeLog:

	PR gdb/22556
	* remote.c (remote_thread_name): Return NULL if name is empty.
	(remote_threads_extra_info): Return NULL if extra info is empty.