fix: use try-with-resources in ProjectBasedTests.isEmpty()

Fixes PMD UseTryWithResources violation introduced during Xtend-to-Java
migration, now that upstream enabled this rule.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: joaodinissf

com.avaloq.tools.ddk.check.core.test/src/com/avaloq/tools/ddk/check/core/test/ProjectBasedTests.java

@@ -61,17 +61,12 @@ public void initializeTestProject() {
   }
 
   private boolean isEmpty(final IFile file) throws CoreException {
-    final InputStream s = file.getContents();
-    try {
+    try (InputStream s = file.getContents()) {
       return s.read() < 0;
+    } catch (CoreException e) {
+      throw e;
     } catch (Exception e) {
       throw new IllegalStateException(e);
-    } finally {
-      try {
-        s.close();
-      } catch (Exception e) {
-        // ignore
-      }
     }
   }