Allow null-returning do expressions with failOnUnknownTokens

[alloutflo]

Summary

• track whether expression evaluation completed a real method invocation
• allow a null result with failOnUnknownTokens only when the complete expression is a successful method call
• keep unknown methods and property access after a null-returning call as errors
• cover both default and eager execution modes

Fixes #1259.

Testing

• focused FailOnUnknownTokens, DoTag, and eager DoTag tests: 15 passed
• full mvn verify under JDK 23 with Java release 17 and an explicit en_US test locale: 2,220 tests passed, 23 skipped
• Spotless, Checkstyle, SpotBugs, and PMD passed

AI assistance disclosure

OpenAI Codex was used to analyze, implement, and test this change. The diff and checks listed above were verified in the contribution workspace.