Update get_news() to work when no sources are provided.

Currently, `sources=[]` by default, which will become "" once the
list is `joined` to form a csv string. The tiingo api appears to
interpret the empty string as "don't match any sources" instead of
"match all sources". This commit fixes the problem by setting
`sources` to `None` if the list is empty.

Adds unit test for empty new sources list.

Updates HISTORY.rst with note about bug fix.

tiingo/api.py

@@ -338,7 +338,7 @@ class TiingoClient(RestClient):
             'offset': offset,
             'sortBy': sortBy,
             'tickers': tickers,
-            'source': (",").join(sources),
+            'source': (",").join(sources) if sources else None,
             'tags': tags,
             'startDate': startDate,
             'endDate': endDate,