Fix Issue with System-Assigned Managed Identity in Azure Functions (#829

)

For some reason, userId is coming as empty in this block, as reported in issue 815

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/AzureManagedIdentityAuthenticationProvider.cs

@@ -89,7 +89,7 @@ public override async Task<SqlAuthenticationToken> AcquireTokenAsync(SqlAuthenti
                 string objectIdParameter = string.Empty;
 
                 // If user assigned managed identity is specified, include object ID parameter in request
-                if (parameters.UserId != default)
+                if (!string.IsNullOrWhiteSpace(parameters.UserId))
                 {
                     objectIdParameter = $"&object_id={Uri.EscapeDataString(parameters.UserId)}";
                     SqlClientEventSource.Log.TryTraceEvent("AcquireTokenAsync | Identity Object id received and will be used for acquiring access token {0}", parameters.UserId);