Handle null dynamic keys in lookup helper

[curforever]

Fixes #1060.

lookup obj person.id currently throws a NullPointerException when the dynamic key resolves to null. LookupHelper dereferences the parameter with toString() before checking that case.

Treat a null key like an absent lookup parameter: return the original context. This preserves this Java helper's existing fallback for absent parameters and missing/null lookup results. A literal string "null" still names a property; non-null lookups are unchanged.

Validation on Windows / Microsoft OpenJDK 21.0.12 / Maven 3.9.11, with the original Java 17 release target and POM:

• The same 16 regression tests: six NPE failures/errors before, all pass after. Includes direct helper calls and real nested/each/subexpression template rendering, plus existing fallback and false/zero/empty-result controls.
• Changed-file Spotless check passes.
• mvn -B -ff -ntp clean install: 1192 passing, 3 default skipped, zero failures/errors.
• mvn -B -ff -ntp clean package: 1192 passing, 3 default skipped, zero failures/errors.

The Mustache spec submodule was checked out at its pinned gitlink. No POM/test exclusions or skip flags were added; repeated builds are not additional distinct tests. The 3 existing skips are one Mustache partial Failed Lookup spec case and two opt-in benchmark cases. No benchmark result is claimed.

Core Javadoc generation reports No source files for package com.github.jknack.handlebars.cache. The unchanged upstream POM already sets maven.javadoc.failOnError=false, so the default build exits with status 0 and downstream doc-link errors are ignored. Replaying the generated Javadoc arguments against original and fixed sources produced exit status 2 and the same error. Documentation generation is not claimed to pass.

Other OS/JDK matrix cells were not run locally.

Prepared with assistance from OpenAI Codex; validation commands above were run locally.