[5.x] Fix is_external_url modifier with Link fields

[duncanmcclean]

This pull request fixes an issue with the is_external_url modifier where it'd error out when passed a Link field.

The error is actually coming from inside the URL::isExternal method, with the caching changes introduced in #9646. The method is expecting a string be passed to it instead of an ArrayableLink object like is the case w/ Link fields.

This worked fine in v4 because ArrayableLink casts to a string but with the recent caching changes, it's trying to get items in an array using ArrayableLink as a key which doesn't work. I though converting the value to a string before it ends up in isExternal might be the best way to fix it.

Fixes #10026.