test: Cover `IsNull()` for PSBT, PSBTInput, PSBTOutput #35848

The motivation for this PR came while reviewing PR#35553, I noticed on corecheck that the IsNull() functions of PartiallySignedTransaction, PSBTInput, PSBTOutput weren't covered by any test.

This PR adds coverage for all of them.

This cases can be tested with the next diffs that don't cause test failure prior to this PR, but with this new test do:

<details> <summary>Diff for <code>PartiallySignedTransaction</code></summary>

diff --git a/src/psbt.cpp b/src/psbt.cpp
index 8f2e9ab16f..59bb05822d 100644
--- a/src/psbt.cpp
+++ b/src/psbt.cpp
@@ -33,7 +33,7 @@ PartiallySignedTransaction::PartiallySignedTransaction(const CMutableTransaction
 
 bool PartiallySignedTransaction::IsNull() const
 {
-    return inputs.empty() && outputs.empty() && unknown.empty();
+    return inputs.empty() && outputs.empty() && !unknown.empty();
 }

</details>

<details> <summary>Diff for <code>PSBTInput</code></summary>

diff --git a/src/psbt.cpp b/src/psbt.cpp
index 8f2e9ab16f..20c745f7a4 100644
--- a/src/psbt.cpp
+++ b/src/psbt.cpp
@@ -291,7 +291,7 @@ COutPoint PSBTInput::GetOutPoint() const
 
 bool PSBTInput::IsNull() const
 {
-    return !non_witness_utxo && witness_utxo.IsNull() && partial_sigs.empty() && unknown.empty() && hd_keypaths.empty() && redeem_script.empty() && witness_script.empty();
+    return non_witness_utxo && witness_utxo.IsNull() && partial_sigs.empty() && unknown.empty() && hd_keypaths.empty() && redeem_script.empty() && witness_script.empty();
 }

</details>

<details> <summary>Diff for <code>PSBTOutput</code></summary>

diff --git a/src/psbt.cpp b/src/psbt.cpp
index 8f2e9ab16f..fdfdfe903f 100644
--- a/src/psbt.cpp
+++ b/src/psbt.cpp
@@ -530,7 +530,7 @@ void PSBTOutput::FromSignatureData(const SignatureData& sigdata)
 
 bool PSBTOutput::IsNull() const
 {
-    return redeem_script.empty() && witness_script.empty() && hd_keypaths.empty() && unknown.empty();
+    return !redeem_script.empty() && witness_script.empty() && hd_keypaths.empty() && unknown.empty();
 }

</details>

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The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35848.

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Reviews

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

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #35797 (psbt: support output metadata updates before inputs are added by l0rinc)
• #35747 (wallet: Fix FillPSBT failing to sign owned inputs when UTXOs disagree by nervana21)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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🚧 At least one of the CI tasks failed. _{Task tidy: https://github.com/bitcoin/bitcoin/actions/runs/30558606139/job/90925472611} _{LLM reason (✨ experimental): CI failed because clang-tidy reported a bugprone-argument-comment warning-as-error: the comment argument name amount in test/psbt_tests.cpp doesn’t match the parameter name nValueIn.}

<details><summary>Hints</summary>

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

</details>