1102 b -Fix error when writing sub-button entity

[MrBearPresident]

Breaking change

Proposed change

Fixing an error I made, apparently I didn't test thoroughly enough.

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

type: custom:bubble-card
card_type: button
button_type: state
entity: sun.sun
sub_button:
  - entity: sun.sun

When changing the entity before this change it was written to:

type: custom:bubble-card
card_type: button
button_type: state
entity: sun.sun
sub_button:
  - entity: 
      entity: sun.sun

Example printscreens/gif

Before:

After:

Additional information

• This PR fixes or closes issue: None reported yet.
• This PR is related to issue or discussion: /
• Link to documentation pull request: /

Additional documentation needed.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests screenshots/gifs have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for readme.
  No need