Fix PR python/17981

Fix PR python/17981

PR python/17981 notes that gdb.breakpoints() returns None when there
are no breakpoints; whereas an empty list or tuple would be more in
keeping with Python and the documentation.

This patch fixes the bug by changing the no-breakpoint return to make
an empty tuple.

Built and regtested on x86-64 Fedora 23.

2016-05-23  Tom Tromey  <tom@tromey.com>

	PR python/17981:
	* python/py-breakpoint.c (gdbpy_breakpoints): Return a new tuple
	when there are no breakpoints.

2016-05-23  Tom Tromey  <tom@tromey.com>

	* python.texi (Basic Python): Document gdb.breakpoints return.

2016-05-23  Tom Tromey  <tom@tromey.com>

	PR python/17981:
	* gdb.python/py-breakpoint.exp (test_bkpt_basic): Add test for
	no-breakpoint case.