[PHJ2000]

Using {{ 'a' | default }} currently raises a missing-argument error. Make default_value optional and use an empty string when omitted, as specified by Jinja's default filter.

The fallback uses getOrDefault, so an explicitly supplied null keeps its existing behavior. The obsolete minimum-argument check is removed, and the parameter documentation describes the fallback.

Four regression tests cover defined values, undefined values, truthy=True, and the d alias. All four fail before the production change; existing explicit-null tests continue to pass.

Fixes #1339.

Validation on JDK 21, with a local Maven toolchains file pointing to the installed JDK:

• mvn -B -ntp -t ../evidence/maven-toolchains.xml -Dproject.build.jdk.version=21 verify: successful, 2,242 tests, 0 failures/errors, 23 existing skips.
• The build's formatting, Checkstyle, SpotBugs, PMD, and dependency checks passed.
• git diff --check: passed.