Harden XMLSecurityDSig types after PHPStan analysis.

Narrow XPath results to DOMElement before attribute access, correct related PHPDocs, and guard failed queries so static analysis and runtime failure paths stay consistent.

Author: robrichards

CHANGELOG.txt

@@ -124,6 +124,10 @@ Improvements:
 - Update parameter type in XMLSecurityDSig::addReference() and addRefInternal()
 - Tighten XMLSecEnc PHPDoc types for algorithm allowlists, references, and
   encrypt/decrypt return values
+- Harden XMLSecurityDSig DOM handling for static analysis: require DOMElement
+  before attribute access on XPath results, correct locateSignature /
+  reference / staticAdd509Cert PHPDocs, guard failed XPath queries and missing
+  SignatureMethod nodes, and remove dead null comparisons
 
 Bug Fixes:
 - Compact signature template (XMLSecurityDSig 'stripWhitespace' option) removes