In VerticalGapData use SysStaff.bbox().y() for the Y-coordinate instead of SysStaff.y() when calculating the vertical position of the SysStaff because in case of single line staves, SysStaff.y() the returned value includes an offset. This resulted in wrong initial spacing between staves when one of the staves was a single line staff.
SysStaff.bbox().y() returns the uncorrected Y-coordinate which is required to calculate the correct spacing between staves.

PR #7854 is a similar PR for master.

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