Fix valgrind error from gdb.decode_line

Fix valgrind error from gdb.decode_line

PR symtab/12535 points out that gdb.decode_line("") will cause a
valgrind report.

I think the empty linespec does not really make sense.  So, this patch
changes gdb.decode_line to treat a whitespace-only linespec the same
as a non-existing argument.

gdb/ChangeLog
2020-01-14  Tom Tromey  <tom@tromey.com>

	PR symtab/12535:
	* python/python.c (gdbpy_decode_line): Treat empty string the same
	as no argument.

gdb/testsuite/ChangeLog
2020-01-14  Tom Tromey  <tom@tromey.com>

	PR symtab/12535:
	* gdb.python/python.exp: Test decode_line with empty string
	argument.

Change-Id: I1d95812b4b7a21d69a3e9afd05b9e3141a931897