Fix ts error for potentially undefined newFileName (#1564)

## Purpose

We had a ts error on build that wasn't caught by eslint. Apparently we were too clever with an if statement, and had to be more explicit.

```
lib/osf-components/addon/components/file-browser/file-rename-modal/component.ts:31:46 - error TS2532: Object is possibly 'undefined'.

31         return (Boolean(this.newFileName) && this.newFileName.trim() !== this.originalFileName &&
                                                ~~~~~~~~~~~~~~~~

lib/osf-components/addon/components/file-browser/file-rename-modal/component.ts:32:13 - error TS2532: Object is possibly 'undefined'.

32             this.newFileName.trim() !== '');
               ~~~~~~~~~~~~~~~~
```

## Summary of Changes

1. Wrap return in an if statement to check for the existence of newFileName

Author: brianjgeiger

lib/osf-components/addon/components/file-browser/file-rename-modal/component.ts

@@ -28,8 +28,10 @@ export default class FileRenameModal extends Component<Args> {
     }
 
     get isValid() {
-        return (Boolean(this.newFileName) && this.newFileName.trim() !== this.originalFileName &&
-            this.newFileName.trim() !== '');
+        if(this.newFileName) {
+            return (this.newFileName.trim() !== this.originalFileName && this.newFileName.trim() !== '');
+        }
+        return false;
     }
 
     @action