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_legacy_query_analyzer.py

@@ -72,7 +72,7 @@ async def conn(table_factory, temp_db_cursor):
     temp_db_cursor.execute("""CREATE OR REPLACE FUNCTION make_standard_name(name TEXT)
                               RETURNS TEXT AS $$ SELECT lower(name); $$ LANGUAGE SQL;""")
 
-    async with NominatimAPIAsync(environ={}) as api:
+    async with NominatimAPIAsync() as api:
         async with api.begin() as conn:
             yield conn