Prevent errors if PDFFindController.executeCommand is ever called without a state object

Most of the code in `PDFFindController` assumes that a valid `state` always exits, hence it cannot hurt to add a simple check to avoid errors being thrown.

web/pdf_find_controller.js

@@ -111,6 +111,9 @@ class PDFFindController {
   }
 
   executeCommand(cmd, state) {
+    if (!state) {
+      return;
+    }
     const pdfDocument = this._pdfDocument;
 
     if (this._state === null || this._shouldDirtyMatch(cmd, state)) {