do_source: should handle missing arg

[blueyed]

source requires/uses an arg, but causes a SyntaxError currently if it is omitted:

(Pdb++) source
*** SyntaxError: unexpected EOF while parsing
Traceback (most recent call last):
  File "/usr/lib/python3.7/pdb.py", line 1159, in _getval
    return eval(arg, self.curframe.f_globals, self.curframe_locals)
  File "<string>", line 0
    
    ^
SyntaxError: unexpected EOF while parsing