Fix Pages not_found_404 parsing

[jacobswe]

When initializing the Dash object using use_pages=True and pages_folder="" you cannot pass a dash.register_page("not_found_404", layout=CUSTOM_404_LAYOUT). Line 2245 joins on a pages_folder and 404 path making the searched module name ".not_found_404". This small edit fixes the error and allows for passing a 404 page using module name "not_found_404" in the case of pages_folder="". Currently, you can (outside documentation) pass ".not_found_404" with a custom layout.

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Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • Fix filepath parsing in pages 404
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• I have added entry in the CHANGELOG.md
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[jacobswe]

I have been migrating my app to 3.5 and, in using the new pages plugin, I manually registered pages after doing some set up (hence the page_folder=""). I noticed couldn't pass the not_found_404 and get it to register correctly, but drilling into the source code I found this would fix the issue, and that using ".not_found_404" as the module name was a working alternative. Opening this to offer a solution (if it's not purposeful behavior) or alternatively that this would be good to document (if it is purposeful behavior).

[alexcjohnson]

Nice catch @jacobswe! Would you be able to add this case to a test? Perhaps pala001 via get_app?

[jacobswe]

Yep! Will write this and update PR over the weekend.

[alexcjohnson]

💃 Looks great, thanks @jacobswe and @AnnMarieW!

[jacobswe]

Thanks for this, I was having dev environment issues and have been traveling for work.