Fixes the root cause behind #344: POST /api/channels accepts a channel without <preprocessingScript>; the channel deploys "healthy" but corrupts every message to the literal string undefined on the wire.

Root cause

Two independent defects in JavaScriptUtil (full analysis in #344):

1. JavaScriptBuilder.generateScript string-concatenates a null script into the JS wrapper, producing function doScript() { null }. compileAndAddScript caches it as a "custom" preprocessor because its body differs from the default script.
2. executePreprocessorScripts guards results with result != null, but Rhino's Undefined is not Java null — Context.jsToJava coerces it to the string "undefined", which becomes the processed raw and flows to every destination.

Changes

• compileAndAddScript: a null/blank script is now treated as absent — evict from the compiled-script cache and return false (all callers already handle false as "no script").
• executePreprocessorScripts: both result checks (global + channel) now ignore Undefined, the same idiom getPostprocessorResponse already uses.
• New JavaScriptUtilTest: 5 tests exercising the real Rhino path (TDD — each fix developed against a failing test).

Behavior change note

A user preprocessor with no return statement previously corrupted the message body to "undefined"; it now passes the message through unchanged. This is footgun-removal: returning the literal string "undefined" as content is never intentional (return ''; and return null; behave as before).

Verification

• Full :server:test suite green.
• E2E on the patched build (the issue's exact repro XML, MLLP sink logging raw bytes):

The broader ask in #344 — structural validation/normalization in POST /api/channels — is deliberately out of scope here; it deserves its own design discussion. This PR removes the data corruption.