Empty Block Templates output string on front-end stating "Empty Template".

[johnstonphilip]

Describe the bug
If a block template has no content in post_content, instead of seeing a blank page (which would match the template), a string is output on the front-end stating "Empty Template", even for logged-out users.

To reproduce
Steps to reproduce the behavior:

1. On a fresh install, create a Block Based Theme with just a single.html file, and put no content in that file.
2. View the "Hello World" post.
3. See the message that states "Empty template.".
4. Log out.
5. Notice you still see that message.

Expected behavior
I would expect 2 things here:

• If logged-out, I would expect to see a blank page on the front-end, which matches the template.
• If logged-in with admin-level capabilities, an error might appear stating "The single.html template was blank", thought this is likely better shown in wp-admin, rather than on the front-end.

Desktop (please complete the following information):

• OS: MacOS Catalina
• Browser: Chrome
• Version 79.0.3945.130 (Official Build) (64-bit)

Additional context
The code for this is happening here:

gutenberg/lib/template-loader.php

Line 99 in a8e49fc

$_wp_current_template_content = empty( $current_template['template_post']->post_content ) ? __( 'Empty template.', 'gutenberg' ) : $current_template['template_post']->post_content;

• Gutenberg version: master.

[johnstonphilip]

A similar problem occurs with template-parts that are empty:

See where a string is returned if a blank template part is found here:

gutenberg/packages/block-library/src/template-part/index.php

Line 33 in 21304f9

return 'Template Part Not Found';

One additional aside thing is that that message is untranslated.

[johnstonphilip]

One other related issue here is this:

• Create a block template called front-page
• Add a template part, and make it call footer
• Don't place anything in the footer template-part.

You will not get the error message that is intended to show, but a blank page, as the output on the page is simply this:

<!-- wp:template-part {"postId":141,"slug":"footer","theme":"my-theme"} /-->

Since that is not empty, you don't get the error message that was intended.

[carolinan]

For template parts, this can be solved with #28398