uv-resolver: slight tweek to try_universal_markers
Previously, we had copied the behavior of `try_markers` to return
`None` in the case where the marker was always true. I believe this
was done because it somewhat implies that there is no forking
happening. But I find this somewhat strange personally, and instead
flipped this around so that it still returns a marker in that case.
The one call site that is impacted by this is the resolution
graph construction. If we left it as-is, it would end up with
a list of one marker that is always true in some cases. And this
in turn results in writing an empty `resolution-markers` to the
lock file. Probably the output logic should be tweaked instead,
but we leave it alone for now.
