Fix (dis)like, music list extraction due to YouTube changes (again)

YouTube reverted the changes they made that prompted f9f5d5ba.

In case they change their minds again, this adds support for both
formats.

The liberal_update and conservative_update functions needed to be
modified to handle the cases of empty lists, so that
a successfully extracted 'music_list': [{'Author':...},...] will
not be overwritten by 'music_list': [] in the calls to
liberal_dict_update.

Signed-off-by: Jesús <heckyel@hyperbola.info>

youtube/yt_data_extract/common.py

@@ -1,6 +1,7 @@
 import re
 import urllib.parse
 import collections
+import collections.abc
 
 def get(object, key, default=None, types=()):
     '''Like dict.get(), but returns default if the result doesn't match one of the types.
@@ -62,17 +63,40 @@ def multi_deep_get(object, *key_sequences, default=None, types=()):
                 continue
     return default
 
+
+def _is_empty(value):
+    '''Determines if value is None or an empty iterable, such as '' and []'''
+    if value is None:
+        return True
+    elif isinstance(value, collections.abc.Iterable) and not value:
+        return True
+    return False
+
+
 def liberal_update(obj, key, value):
-    '''Updates obj[key] with value as long as value is not None.
-    Ensures obj[key] will at least get a value of None, however'''
-    if (value is not None) or (key not in obj):
+    '''Updates obj[key] with value as long as value is not None or empty.
+    Ensures obj[key] will at least get an empty value, however'''
+    if (not _is_empty(value)) or (key not in obj):
         obj[key] = value
 
 def conservative_update(obj, key, value):
-    '''Only updates obj if it doesn't have key or obj[key] is None'''
-    if obj.get(key) is None:
+    '''Only updates obj if it doesn't have key or obj[key] is None/empty'''
+    if _is_empty(obj.get(key)):
         obj[key] = value
 
+
+def liberal_dict_update(dict1, dict2):
+    '''Update dict1 with keys from dict2 using liberal_update'''
+    for key, value in dict2.items():
+        liberal_update(dict1, key, value)
+
+
+def conservative_dict_update(dict1, dict2):
+    '''Update dict1 with keys from dict2 using conservative_update'''
+    for key, value in dict2.items():
+        conservative_update(dict1, key, value)
+
+
 def concat_or_none(*strings):
     '''Concatenates strings. Returns None if any of the arguments are None'''
     result = ''