fix: omit KeyInfo when there is no content for it

[cjbarth]

Closes #596

XMLDSig's schema requires KeyInfo (4.5) and X509Data (4.5.4) to each contain at least one child element. Signing could produce either one empty:

Configuration	Before	After
publicCert has no BEGIN CERTIFICATE block, e.g. a PEM public key	<KeyInfo><X509Data/></KeyInfo>	no KeyInfo
keyInfoAttributes set, no publicCert	<KeyInfo Id="key">null</KeyInfo>	no KeyInfo

Changes

• SignedXml.getKeyInfoContent returns null when publicCert contains no certificate. Its JSDoc already defines null as "no KeyInfo element should be included".
• getKeyInfo() emits KeyInfo only when getKeyInfoContent returns content. keyInfoAttributes alone no longer produce an element.
• The README's extension-point section says what happens when there is no content.

Compatibility

This targets v6.2 and does not throw. The issue notes that throwing when keyInfoAttributes are set with no content would be more explicit, but it would break configurations that sign today, so that choice is left for 7.0.

A caller who set keyInfoAttributes with no content and added a reference to KeyInfo now gets the existing "xpath cannot be signed because it was not found" error instead of a signature over <KeyInfo>null</KeyInfo>.

Tests

Two regression tests sign with each configuration and check that the signature has no KeyInfo. Both failed before the fix because a KeyInfo element was present.

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📥 Commits

Reviewing files that changed from the base of the PR and between fb8e7c6 and 7633e4d.

📒 Files selected for processing (3)

• README.md
• src/signed-xml.ts
• test/signature-unit-tests.spec.ts

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.96%. Comparing base (f5c4d22) to head (7633e4d).
⚠️ Report is 20 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #597      +/-   ##
==========================================
+ Coverage   75.95%   79.96%   +4.01%     
==========================================
  Files           9        9              
  Lines        1048     1118      +70     
  Branches      273      283      +10     
==========================================
+ Hits          796      894      +98     
+ Misses        144      129      -15     
+ Partials      108       95      -13     

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