Always-hidden staff incompatible with multimeasure rests

"Always" hidden staff appear in systems that have multimeasure rests in 4.0. This was not happening in 3.6 and complicates migrating files to 4.0.

Steps to reproduce:
1. Add an instrument with two staffs
2. On one staff, set "Hide when empty" to "Always" in Staff/Part properties
3. On the non-hidden staff, write something that has multiple consecutive measures of rest
4. Enable multimeasure rests. The hidden staff reappears in the system with the multimeasure rest.

On the attached file, the second staff is set to always hide, while the first staff remains on Auto. Nevertheless, staff 2 reappears in system 4 due to the multimeasure rest spanning measures 8-9.