When the numTweaks is zero, we should return a nil instead of
initializing an empty map as we'd get the following error,
```
Diff:
--- Expected
+++ Actual
@@ -11007,4 +11007,3 @@
},
- BreachedHtlcTweaks: (contractcourt.htlcTapTweaks) {
- },
+ BreachedHtlcTweaks: (contractcourt.htlcTapTweaks) <nil>,
```
In this commit, we add a new taproot specific briefcase to store the
control block and tap tweaks for all taproot outputs. We chose this
route as many of the existing fields are serialized in line, so we
aren't able to serialize this new taproot specific information in the
existing briefcase.
