Fix empty heading anchor id leaving an empty id attribute on heading html tags

[andrewserong]

Description

Resolves: #11663

This PR resolves the bug where an empty anchor id on a heading block would result in a stray empty id attribute on the heading html tag. This solution updates the editor anchor hook to check that the anchor id attribute is not an empty string before saving it as additional props. Also adds a test to cover this behaviour.

How has this been tested?

• Added additional unit test for the anchor addSaveProps method to check that an empty string passed in to extraProps does not result in an id being created.
• Manually tested in Chrome 70.0.3538.77 and Safari Version 12.0 (13606.2.11) on macOS High Sierra 10.13.6.

Steps to reproduce the behaviour (from issue #11663):

• Insert a heading block and type some text
• In the Advanced panel, set an HTML Anchor attribute
• Empty the HTML Anchor field
• Switch to Code Editor.

Screenshots

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[gziolo]

The same issue was fixed with a very similar change in #12728 by @saleehk. I'm closing this one as a duplicate. Thank you for your time spend on this contribution.