Transforming quote without citation to heading produces an extra empty quote

[csvarga]

Bug description:
In the editor when transforming from quote block type to heading block type, and the quote has quote content but no citation, it creates a new empty quote block type that seems unnecessary.

Steps to reproduce the behavior:

1. Create a quote block type
2. Write something in the quote content area, and leave citation party empty
3. Click Change block type
4. Transform To: click Heading

What we start with:
One quote with quote content and no citation

What we should end up with:
One heading block with the quote content

What we do end up with:
One heading block (with the quote content) and another quote block with no content at all

Tested on:

• Desktop
• OS: macOS 10.14.3
• Browser: Chrome 72.0.3626.96
• WordPress: version 5.1
• Gutenberg: version 5.2.0

Other notes:
I found #5052 describing the issue, then #5088 supposedly fixed it, but the logic that has been implemented and is live now still seems strange. It is not clear why the extra quote block should be created, what use case that serves.