fix: issue #2495

[michalismeng]

Fix issue #2495.

Changes:

• Add a default block only when a placeholder is defined.
• In renderer, inserting an empty block when zero blocks are given, results in an extra block being rendered. In readonly mode this is empty. Fix this behavior.

Outcome of this PR:

• Display the placeholder text, only if defined (whether in readonly mode or not).
• Do not add an extra empty paragraph in readonly mode.