gdb/python: don't allow the user to delete window title attributes

gdb/python: don't allow the user to delete window title attributes

There's a bug in the python tui API.  If the user tries to delete the
window title attribute then this will trigger undefined behaviour in
GDB due to a missing nullptr check.

gdb/ChangeLog:

	* python/py-tui.c (gdbpy_tui_set_title): Check that the new value
	for the title is not nullptr.

gdb/testsuite/ChangeLog:

	* gdb.python/tui-window.exp: Add new tests.
	* gdb.python/tui-window.py (TestWindow) <__init__>: Store
	TestWindow object into global the_window.
	<remote_title>: New method.
	(delete_window_title): New function.