Fix: add call to ensure required libraries in get_conn_params

added `ensure_required_libs(module)` to `get_conn_params` to ensure a meaningful error is thrown when psycopg2 is not installed. Without this change the error was; `AttributeError: 'NoneType' object has no attribute '__version__'`.

Fixes ansible-collections#252

plugins/module_utils/postgres.py

@@ -177,6 +177,9 @@ def get_conn_params(module, params_dict, warn_db_default=True):
     Kwargs:
         warn_db_default (bool) -- warn that the default DB is used (default True)
     """
+
+    ensure_required_libs(module)
+
     # To use defaults values, keyword arguments must be absent, so
     # check which values are empty and don't include in the return dictionary
     params_map = {