Code/getfslineno: keep empty co_filename

Previously this would be turned via `py.path.local("")` into the current
working directory.

This appears to be what `fspath = fn and py.path.local(fn) or None`
tries to avoid in `getfslineno`'s `TypeError` handling already, if
`Code` would raise it.

src/_pytest/_code/code.py

@@ -72,6 +72,8 @@ class Code:
         """ return a path object pointing to source code (or a str in case
         of OSError / non-existing file).
         """
+        if not self.raw.co_filename:
+            return ""
         try:
             p = py.path.local(self.raw.co_filename)
             # maybe don't try this checking