fix: make close() sync (motor close is sync); guard doc[key] on missing fields

close() was async but only called motor's synchronous client.close(), misleading callers into awaiting it; it is now a plain def and the docstring + README examples drop the await. get_document_hashmap/get_document_fields indexed doc[key] unguarded, so a doc missing the field raised KeyError that the broad except swallowed, conflating 'no documents' with 'field absent'; both now skip docs lacking the key.

Signed-off-by: disqualifier <dev@disqualifier.me>

README.md

@@ -28,7 +28,7 @@ from mongo import Mongo
 
 db = Mongo(conn_string, database)          # attach as bot.db / app.db
 users = await db.get_documents("users", {"active": True})
-await db.close()                            # on shutdown
+db.close()                                  # on shutdown (sync)
 ```
 
 **Module proxy (back-compat)** — arm once, then call bare: