Fixes issue 2029 - Fix placeholderVisible state depending on isEmpty on mode switch

[r4pt0s]

1. Why is this change necessary?

To provide a meaningful user experience with expected behaviour for merchants.

2. What does this change do, exactly?

It updates the placeholderVisible state properly depending on mode switch (code editor mode => text editor mode)

3. Describe each step to reproduce the issue or behaviour.

See issue nr #2029

4. Please link to the relevant issues (if any).

#2029

5. Checklist

• I have written tests and verified that they fail without my change
• [ X] I have squashed any insignificant commits
• [X ] I have created a changelog file with all necessary information about my changes
• I have written or adjusted the documentation according to my changes
• This change has comments for package types, values, functions, and non-obvious lines of code
• [ X] I have read the contribution requirements and fulfil them.

[CLAassistant]

All committers have signed the CLA.

[shopwareBot]

Hello,

thank you for creating this pull request.
I have opened an issue on our Issue Tracker for you. See the issue link: https://issues.shopware.com/issues/NEXT-17045

Please use this issue to track the state of your pull request.

[NiklasLimberg]

Thanks for your contribution 🎉