Allow empty wiki pages

What does this MR do?

In this MR we're removing the constraint for wiki pages to have content. This aligns the UI with the Git workflow, because users can push empty wiki pages through it and, because of this restriction, the validator raises some errors in the background job.

The changes are backend related ones. We still need to change the UI to allow empty wiki pages.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Related to #221061 (closed)