KeywordBear: Handle empty keywords

If setting keyword is used in coafile and not assigned, then keywords
is an empty list, which leads to construction of regex `r'()'` which
matches all the letters, hence yielding false results.

Fixes coala#1689

bears/general/KeywordBear.py

@@ -96,13 +96,14 @@ def run(self,
         '''
         comments = list(_get_comments(dependency_results))
 
-        simple_keywords_regex = re.compile(
-            '(' + '|'.join(re.escape(key) for key in keywords) + ')',
-            re.IGNORECASE)
-
-        message = "The line contains the keyword '{}'."
-        yield from self.check_keywords(filename, file, comments,
-                                       simple_keywords_regex, message)
+        if keywords:
+            simple_keywords_regex = re.compile(
+                '(' + '|'.join(re.escape(key) for key in keywords) + ')',
+                re.IGNORECASE)
+
+            message = "The line contains the keyword '{}'."
+            yield from self.check_keywords(filename, file, comments,
+                                           simple_keywords_regex, message)
 
         if regex_keyword is not '':
             regex = re.compile(regex_keyword)

tests/general/KeywordBearTest.py

@@ -239,3 +239,12 @@ def test_wrong_language(self):
                 self.assertIn(log.output[0],
                               'ERROR:root:coalang specification'
                               ' for anything not found.')
+
+    def test_empty_keywords_list(self):
+        self.section.append(Setting('keywords', ''))
+
+        text = ['bears = KeywordBear\n']
+
+        with execute_bear(self.uut, filename='F', file=text,
+                          dependency_results=self.dep_results) as result:
+            self.assertEqual(len(result), 0)