PropertyExpression zoom evaluation should not assume zoom is non-constant

[brunoabinader]

We shouldn't need to worry a zoom value is non-constant when evaluating according to zoom. This causes a crash when e.g. creating a Formatted expression and evaluating this according to zoom. This can happen if e.g. the symbol layer is the leader.

[ChrisLoer]

I think this assert here is intentional and correct. We want to catch cases where we accidentally create PropertyExpressions that can evaluate constant properties -- we should be using constant values instead of PropertyExpressions in those cases.