[FIX] data/io.py Metadata file not saved anymore when it is empty

Issue
Do not save the metadata file (when saving the data) if this is empty (0 Bytes)

Description of changes
Change if statement and add third condition. It implies that attribute should not be empty.

Author: jerneju

Orange/data/io.py

@@ -373,7 +373,7 @@ def write(cls, filename, data):
 
     @classmethod
     def write_table_metadata(cls, filename, data):
-        if isinstance(filename, str) and hasattr(data, 'attributes'):
+        if isinstance(filename, str) and getattr(data, 'attributes', None):
             if all(isinstance(key, str) and isinstance(value, str)
                    for key, value in data.attributes.items()):
                 with open(filename + '.metadata', 'w') as f:

Orange/tests/test_tab_reader.py

@@ -206,3 +206,23 @@ def test_data_name(self):
         table2 = TabReader(table1.__file__).read()
         self.assertEqual(table1.name, 'iris')
         self.assertEqual(table2.name, 'iris')
+
+    def test_metadata(self):
+        tempdir = tempfile.mkdtemp()
+        table = Table("titanic")
+        table.attributes = OrderedDict()
+        table.attributes["a"] = "aa"
+        table.attributes["b"] = "bb"
+        fname = path.join(tempdir, "out.tab")
+        TabReader.write_table_metadata(fname, table)
+        self.assertTrue(path.isfile(fname + ".metadata"))
+        shutil.rmtree(tempdir)
+
+    def test_no_metadata(self):
+        tempdir = tempfile.mkdtemp()
+        table = Table("titanic")
+        table.attributes = OrderedDict()
+        fname = path.join(tempdir, "out.tab")
+        TabReader.write_table_metadata(fname, table)
+        self.assertFalse(path.isfile(fname + ".metadata"))
+        shutil.rmtree(tempdir)