restrict use of os.environ in Configuration

Only use the OS environment, when the environ parameter is set
to None. Currently it would use the OS env on an empty dict.

test/python/api/search/test_icu_query_analyzer.py

@@ -38,7 +38,7 @@ async def conn(table_factory):
     table_factory('word',
                   definition='word_id INT, word_token TEXT, type TEXT, word TEXT, info JSONB')
 
-    async with NominatimAPIAsync(environ={}) as api:
+    async with NominatimAPIAsync() as api:
         async with api.begin() as conn:
             yield conn