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