[illusionoftalha]

Fixes #277.

ClasspathResolver.getReader dereferences the thread context classloader even when it is null. A context classloader with no matching resource already falls back to the resolver's defining classloader; use that same loader up front when the context loader itself is absent. This retains the existing resource and directory checks.

Add three JUnit regressions for a null context classloader: finding an existing template, returning null for a missing template, and rejecting a directory. Each test restores the original context classloader in a finally block.

Validation on Windows / JDK 17:

• Before the source fix, all three new tests throw NullPointerException.
• mvn -DCI=true -pl compiler -am -Dtest=ClasspathResolverTest -Dsurefire.failIfNoSpecifiedTests=false test: 19 tests passed.
• The broader compiler run executes 288 tests, with seven failures and eight skips. I repeated it with the original resolver from base commit 023374b: the same seven failures remain, plus the three new classloader errors. The seven unchanged failures are newline-sensitive assertions in DynamicNamesTest, InterpreterTest, Issue278Test, MultipleRecursivePartialsTest and PreTranslateTest.
• A full mvn -DCI=true package attempt stops at those compiler test failures, so the remaining reactor modules were not validated. The CI flag avoids the project's unrelated benchmark loops.
• git diff --check passes. No test resources are included in the patch.