clipboard: fix clipboard block's result being undefined sometimes (#1346

)

May resolve [a recent conversation in the TurboWarp Discord
server](https://discord.com/channels/837024174865776680/1132437506655268926/1215424597730787368)

In some cases, the Clipboard extension's `clipboard` block can return
undefined. The most reproducible case I could find is using a browser
that supports the block (e.g Chrome), accepting TurboWarp's clipboard
permission prompt but *denying* the *browser's* clipboard permission
prompt.
This PR should fix it. (At least, it fixes the reproducible allow+deny
behavior.)

extensions/clipboard.js

@@ -105,12 +105,16 @@
 
     clipboard() {
       if (navigator.clipboard && navigator.clipboard.readText) {
-        return Scratch.canReadClipboard().then((allowed) => {
-          if (allowed) {
-            return navigator.clipboard.readText();
-          }
-          return "";
-        });
+        return Scratch.canReadClipboard()
+          .then((allowed) => {
+            if (allowed) {
+              return navigator.clipboard.readText() ?? "";
+            }
+            return "";
+          })
+          .catch(() => {
+            return "";
+          });
       }
       return "";
     }