fix: properly handle visible whitespaces in empty blocks

[idebenone]

Problem

Improper handling of visible whitespaces causing unwanted behaviour such as:

• unable to delete trailing whitespaces in empty blocks.
• caret jumping to the end of placeholder in paragraph block.

Solution

Update the isNodeEmpty() to handle visible whitespaces by removing the method call trim() in nodeText.trim().length as it would remove the trailing whitespaces if any exist which in turn falsely marks a block as empty. And because of this the whitespaces are unable to be removed.

[neSpecc]

I think, we should add tests for these 2 cases

• backspace should remove trailing whitespaces in empty blocks
• whitespaces in empty block should hide the placeholder

[neSpecc]

Be aware, don't delete submodules from the repo