fix: base64 encoded tiny editor as iframe src bug

[0-don]

Related Ticket:

Description of Changes: if tinymce is shown in an iframe with src being data url it breaks, because it doesnt have a real url so the path spliting is broken

Pre-checks:

• [x] Changelog entry added
• [x] Tests have been added (if applicable)
• [x] Branch prefixed with feature/ for new features (if applicable)

Review:

• [x] Milestone set
• [x] Review comments resolved

GitHub issues (if applicable):

[metricjs]

HI @Don-Cryptus,

Thanks for this PR. Could you please give us some more information about what the issue is and what behaviour it causes? A replication case would be great - would it be possible for you to create a Fiddle?

[0-don]

I cant exactly replicate it on fiddle, but it would look something like this. I made a new github repo where I am using webpack to put everything into one html file, convert the file to base64 and put it as iframe src. github repo

my example repo works like this open the tinymce iframe.html select either the fixed or original html and it will set the source of the iframe with the encoded string. My forked repo has the fix already implemented so the fixed/index.html shows the working editor but the orignal/index.html will throw the error.