v3dv: don't always assert that we have an active job

There are some scenario where this won't happen and don't imply a bug.
For example, if we find a pipeline barrier, we will finish the current
job automatically and won't start a new one. There may be other
scenarios where we may want to do the same.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>
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