Fix compilation of pk()-only policies into tr() descriptors

[shesek]

Before this fix, the added test failed with: Unexpected("Empty Miniscript compilation")

[tcharding]

Thanks for the contribution!

Can you put the unit test as a separate patch after the fix please so that during review I can swap them around and verify the failure.

[shesek]

@tcharding Yes for sure, I updated my branch.

[tcharding]

Bother, the CI fail is unrelated to this PR. I hit the same thing elsewhere yesterday, I'll push a PR to fix it (it requires dependency pinning).

[tcharding]

I verified the unit test does indeed fail and the fix does fix the test. I have no view on the sanity of the change, requires @apoelstra or @sanket1729 who actually know this crate. I'm just the code monkey :)

Thanks for the contribution.

[sanket1729]

It's been a while since I looked at MSRV things. What is the current rust-bitcoin MSRV?

The CI is failing at

error: package cc v1.0.96 cannot be built because it requires rustc 1.63 or newer, while the currently active rustc version is 1.56.1

[tcharding]

Will need rebasing once #678 goes in.