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.

testing/code/test_source.py

@@ -524,6 +524,14 @@ def test_getfslineno() -> None:
     B.__name__ = "B2"
     assert getfslineno(B)[1] == -1
 
+    co = compile("...", "", "eval")
+    assert co.co_filename == ""
+
+    if hasattr(sys, "pypy_version_info"):
+        assert getfslineno(co) == ("", -1)
+    else:
+        assert getfslineno(co) == ("", 0)
+
 
 def test_code_of_object_instance_with_call() -> None:
     class A: