Fix identifers validation against null next node

What does this MR do?

Fixes a subtle issue in the static site editor when parsing identifier syntax that includes inline code.

Screenshots

The previous solution didn't account for the fact that the next node could be null while still having a valid literal value. This update resolves this case.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #221082 (closed)