Fix #911 handle perforce files (#400)

- Additional handling of document events with "perforce:" or other SCM strings
- Much of the handling had been added already, but adding in additional validation and ensuring that everywhere `Workspace.GetFile` is called we add a not-null check to avoid null reference errors

src/Microsoft.SqlTools.ServiceLayer/QueryExecution/QueryExecutionService.cs

@@ -693,7 +693,10 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
             {
                 // Get the document from the parameters
                 ScriptFile queryFile = WorkspaceService.Workspace.GetFile(docRequest.OwnerUri);
-
+                if (queryFile == null)
+                {
+                    return string.Empty;
+                }
                 // If a selection was not provided, use the entire document
                 if (docRequest.QuerySelection == null)
                 {