Fix error with empty filename

tests/samples/sample_no_filename.diff

@@ -0,0 +1,6 @@
+--- 
++++ 
+@@ -1,2 +1,2 @@
+-foo
+ bar
++foobar

tests/test_parser.py

@@ -534,3 +534,11 @@ def test_git_renaming(self):
         # by unidiff are the same
         with codecs.open(file_path, 'r', encoding='utf-8') as diff_file:
             self.assertEqual(diff_file.read(), str(res))
+
+    def test_no_filename_in_header(self):
+        tests_dir = os.path.dirname(os.path.realpath(__file__))
+        file_path = os.path.join(tests_dir, 'samples/sample_no_filename.diff')
+        with codecs.open(file_path, 'r', encoding='utf-8') as diff_file:
+            res = PatchSet(diff_file)
+
+        self.assertEqual(' ', res.modified_files[0].source_file)

unidiff/constants.py

@@ -30,9 +30,9 @@
 
 
 RE_SOURCE_FILENAME = re.compile(
-    r'^--- (?P<filename>"?[^\t\n]+"?)(?:\t(?P<timestamp>[^\n]+))?')
+    r'^---( ?)(?P<filename>"?[^\t\n]+"?)(?:\t(?P<timestamp>[^\n]+))?')
 RE_TARGET_FILENAME = re.compile(
-    r'^\+\+\+ (?P<filename>"?[^\t\n]+"?)(?:\t(?P<timestamp>[^\n]+))?')
+    r'^\+\+\+( ?)(?P<filename>"?[^\t\n]+"?)(?:\t(?P<timestamp>[^\n]+))?')
 
 
 # check diff git line for git renamed files support

unidiff/patch.py

@@ -549,7 +549,7 @@ def _parse(
             if is_hunk_header:
                 patch_info = None
                 if current_file is None:
-                    raise UnidiffParseError('Unexpected hunk found: {line}')
+                    raise UnidiffParseError(f'Unexpected hunk found: {line}')
                 current_file._parse_hunk(
                     line,
                     enumerated_diff,