rest: use empty() instead of size()/length() comparisons #35423

Replace several .size() == 0, .size() > 0, .length() == 0 and .length() > 0 emptiness checks in src/rest.cpp with the more idiomatic and self-documenting empty() / !empty().

This is a behaviour-preserving cleanup.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35423.

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Reviews

See the guideline for information on the review process. A summary of reviews will appear here.

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Please don't open new pull requests just for tiny refactors, they are a drag on our integration testing infrastructure, maintainer time and reviewer time. See https://github.com/bitcoin/bitcoin/blob/master/CONTRIBUTING.md#refactoring:

Trivial pull requests or pull requests that refactor the code with no clear benefits may be immediately closed by the maintainers to reduce unnecessary workload on reviewing.

There are many more significant ways to contribute to Bitcoin -- for example, look for good first issues.

There are many more significant ways to contribute to Bitcoin -- for example, look for good first issues.

(The "good first issues" label and template have been removed going forward: #35399 and #35400 -- here is an alternative suggestion: #35399 (comment).)