use a multiline string for the empty-import message

Review feedback on #4: text block instead of concatenation. The message now reads
over two lines, which also matches how the other rules format multi-line output.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: SirCotare

src/main/java/it/aboutbits/archunit/toolbox/rule/base/AnalyzedPackagesMustContainClassesArchRule.java

@@ -18,10 +18,9 @@ public interface AnalyzedPackagesMustContainClassesArchRule {
     @ArchTest
     default void analyzed_packages_must_contain_classes(JavaClasses classes) {
         if (classes.isEmpty()) {
-            throw new AssertionError(
-                    "No classes were imported, so none of the architecture rules checked anything. "
-                            + "Verify the packages passed to @AnalyzeClasses."
-            );
+            throw new AssertionError("""
+                    No classes were imported, so none of the architecture rules checked anything.
+                    Verify the packages passed to @AnalyzeClasses.""");
         }
     }
 }