Improve `Tf.Status` python binding

[roggiezhang-nv]

Description of Issue

Tf.Status accepts a verbose argument in python https://github.com/PixarAnimationStudios/OpenUSD/blob/59992d2178afcebd89273759f2bddfe730e59aa8/pxr/base/tf/__init__.py#L181

This has no equivalent in C++, so the binding attempts to set a "null context" by passing empty strings for module, functio, and filename.

Those arguments are passed to Tf_PythonCallContext

https://github.com/PixarAnimationStudios/OpenUSD/blob/59992d2178afcebd89273759f2bddfe730e59aa8/pxr/base/tf/wrapStatus.cpp#L32

which joins the module & function name without checking if they are empty

https://github.com/PixarAnimationStudios/OpenUSD/blob/release/pxr/base/tf/pyCallContext.cpp#L42

resulting in a TfCallContext with a function "." instead of the intended "" (or ideally None/nullptr).

This is highly confusing on the c++ side (e.g. in a TfDiagnosticMgr Delegate implementation).

Hopefully this is a non-contentious fix to check for empty string before joining.

But I also wonder if it wouldn't be better to use TfCallContext::hide() instead so c++ callsites could check diagnostic.GetContext().IsHidden() rather than diagnostic.GetSourceFunction().empty()

Steps to Reproduce

System Information (OS, Hardware)

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Build Flags

[roggiezhang-nv]

@nvmkuruc for vis.

[jesschimein]

Filed as internal issue #USD-10179