[MU3 Backend] ENG-18-adjacent Remove left FretDiagram spacer if first beat of measure

[iveshenry18]

Resolves: ENG-18: Fretboard diagrams need much improvement

This commit changes the spacer code in ChordRest::shape to prevent a
left space from being created in the first beat of a measure. This
spacer was causing an unnecessary gap between the barline and the first
beat.

• I signed CLA
• I made sure the code in the PR follows the coding rules
• I made sure the code compiles on my machine
• I made sure there are no unnecessary changes in the code
• I made sure the title of the PR reflects the core meaning of the issue you are solving
• I made sure the commit message(s) contain a description and answer the question "Why do those changes fix that particular issue?" or "Why are those changes really necessary as improvements?"
• I made sure the commit message title starts with "fix #424242:" if there is a related issue
• I created the test (mtest, vtest, script test) to verify the changes I made

[iveshenry18]

Folded into #8636