Block filler is not removed when converting <pre><code>

[tomalec]

This is a follow-up from #7240

📝 Provide detailed reproduction steps (if any)

1. Open editor with an empty code block.
2. (optional) Execute editor.getData() // "<pre><code>&nbsp;</code></pre>"
3. Execute editor.setData( "<pre><code>&nbsp;</code></pre>" )

✔️ Expected result

You should get the editor with an empty code block (, where you cannot move the caret between any characters).

❌ Actual result

The editor is created with one non-breaking space inside code block;

📃 Other details

• Browser: Chrome, FF
• OS: Win10
• CKEditor version: master (@ad227917a6)
• Installed CKEditor plugins: code-block

Undesired, but expected behavior is covered by this test:

ckeditor5/packages/ckeditor5-code-block/tests/codeblockediting.js

Lines 892 to 900 in 40629c5

	// Undesired by expected. There is an issue with identifying the correct filler type.
	// <code> is inline, so dom-to-view converter expects an inline filler.
	it( 'should convert pre > code with only &nbsp; inside to a codeBlock with &nbsp;', () => {
	editor.setData( '<pre><code>&nbsp;</code></pre>' );
	expect( getModelData( model ) ).to.equal(
	'<codeBlock language="plaintext">[]\u00a0</codeBlock>'
	);
	} );

More details, with investigation results at #7240 (comment)

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