Allow 'null' to be cached.

This prevents repeated lookups in the database for stored procedures with no parameters.

Author: bgrainger

src/MySqlConnector/MySql.Data.MySqlClient/MySqlConnection.cs

@@ -311,9 +311,10 @@ internal async Task<CachedProcedure> GetCachedProcedure(IOBehavior ioBehavior, s
 				return null;
 
 			CachedProcedure cachedProcedure;
+			bool foundProcedure;
 			lock (cachedProcedures)
-				cachedProcedures.TryGetValue(normalized.FullyQualified, out cachedProcedure);
-			if (cachedProcedure == null)
+				foundProcedure = cachedProcedures.TryGetValue(normalized.FullyQualified, out cachedProcedure);
+			if (!foundProcedure)
 			{
 				cachedProcedure = await CachedProcedure.FillAsync(ioBehavior, this, normalized.Schema, normalized.Component, cancellationToken).ConfigureAwait(false);
 				lock (cachedProcedures)