Fix the return type of walkInvocationTree's callback to allow void returns as intended

[Shaptic]

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[github-actions]

Size Change: +39.5 kB (+1.23%)

Total Size: 3.25 MB

Filename	Size	Change
dist/stellar-base.js	2.39 MB	+29.9 kB (+1.27%)
dist/stellar-base.min.js	867 kB	+9.6 kB (+1.12%)

_{compressed-size-action}

[sreuland]

changes look good, can approve, but for consideration, wondering if it's time to introduce ts test to assert these will work from client before releasing, same observation noted on https://github.com/stellar/js-stellar-base/pull/766/files#r1729557085

[Shaptic]

@sreuland good suggestion 🙏 generally the type tests don't help much but you can see in 580b2b8 that this actually uncovered the fact that there is in fact a void return value (rather than undefined) which is great 👏 please review again!

[sreuland]

lgtm, nice work!