An unconfigured HdxColorizeTask never converges

[marktucker]

Description of Issue

When an HdxTaskController creates an HdxColorizeTask, but the colorize task is not configured with a specific aov buffer, the task will never converge, so the task controller will never converge, so any progressive rendering will continue forever.

Steps to Reproduce

1. Create an HdxTaskController but with no aov buffer specified through SetViewportRenderOutput.
2. Note that the HdxColorizeTask is create, but with an empty _aovBufferPath, which results in a null _aovBuffer.
3. In the Execute method, the null _aovBuffer results in an early exit which does not set the _converged flag.
4. Any subsequent call to HdxTaskController::IsConverged will return fals because the colorize task can never converge.

This is admittedly not a standard use case, but it is part of our use case. And it makes sense to me that a colorize task that has not been asked to do anything should always be marked as converged, since it by definition has completed all its work.

[jilliene]

Filed as internal issue #USD-5374

[tcauchois]

Fixed by PR 875.