fix(core): Invoke event listener in windows safely to avoid causing uncaught errors in windows that have loaded external urls

[0rvar]

When opening a tauri window with an External URL, events sent with emit_all, including internal events like tauri://window-created, are still delivered.
Since these windows have not loaded the necessary code, the invocation fails and causes an uncaught javascript error in the window's webview.

Functionally speaking, this is fine, but it's not very neat and has some noticeable effects:

• These errors will show up in the target websites error logging, if such is implemented.
• During development of apps with an error view, these errors will show up as a big modal covering the content - see screenshot below.

This PR adds a simple safeguard that checks that the listener function exists before calling it.

What kind of change does this PR introduce?

• Bugfix

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue #___
• No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary

Other information

Implementation approach briefly discussed here: https://discord.com/channels/616186924390023171/986184094050316358/1038527853236592661

[amrbashir]

is there a reason to use var instead of let?