YapfBear: Ignore file if it is zero-byte file

Yapf cannot handle zero-byte files correctly, because it adds a newline
into the file when correcting. Hence we ignore zero byte files
completely as they anyway do not have code to format.

Fixes #739

bears/python/YapfBear.py

@@ -113,6 +113,11 @@ def run(self, filename, file,
         :param based_on_style:
             The formatting style to be used as reference.
         """
+        if not file:
+            # Yapf cannot handle zero-byte files well, and adds a redundent
+            # newline into the file. To avoid this, we don't parse zero-byte
+            # files as they cannot have anything to format either.
+            return
 
         options = """
 [style]

tests/python/YapfBearTest.py

@@ -24,6 +24,13 @@ def test_valid(self):
                             ["x = {  'a':37,'b':42,\n", "'c':927}\n", '\n',
                              "y = 'hello ''world'\n"], valid=False)
 
+    def test_eof_handling(self):
+        self.check_validity(self.uut, [], valid=True)
+        self.check_validity(self.uut, [''], valid=True)
+        self.check_validity(self.uut, ['a = 2\n'], valid=True)
+        self.check_validity(self.uut, ['a = 2'], valid=True)
+        self.check_validity(self.uut, ['\n'], valid=True)
+
     def test_valid_python_2(self):
         self.check_validity(self.uut, ['print 1\n'], valid=True)