[JSInterp] Use `,` for join() with null/undefined argument

Eg: [1,2,3].join(null) -> '1,2,3'

youtube_dl/jsinterp.py

@@ -1208,7 +1208,8 @@ class JSInterpreter(object):
                 elif member == 'join':
                     assertion(isinstance(obj, list), 'must be applied on a list')
                     assertion(len(argvals) <= 1, 'takes at most one argument')
-                    return (',' if len(argvals) == 0 else argvals[0]).join(
+                    return (',' if len(argvals) == 0 or argvals[0] in (None, JS_Undefined)
+                            else argvals[0]).join(
                                 ('' if x in (None, JS_Undefined) else _js_toString(x))
                                 for x in obj)
                 elif member == 'reverse':