Add unit tests for ArrayUtils, Preconditions and ExtendedSafeRunner

This commit improves code coverage for utility classes in the
org.moreunit.core bundle.

Changes:
- Added ArrayUtilsTest.java
- Added PreconditionsTest.java (covering all methods and exception cases)
- Added ExtendedSafeRunnerTest.java (including exception handling)
- Improved StringsTest.java with coverage for emptyArray()

These tests ensure that core utilities are robust and well-verified.

Co-authored-by: RoiSoleil <3462260+RoiSoleil@users.noreply.github.com>

Author: google-labs-jules[bot]

org.moreunit.core.test/test/org/moreunit/core/util/ExtendedSafeRunnerTest.java

@@ -70,6 +70,8 @@ public String run(String element) throws Exception
             public void handleException(Throwable throwable, String element)
             {
                 exceptionHandled[0] = true;
+                assertThat(element).isEqualTo("input");
+                assertThat(throwable.getMessage()).isEqualTo("failed");
             }
         });
 

org.moreunit.core.test/test/org/moreunit/core/util/StringsTest.java

@@ -127,4 +127,10 @@ public void join_should_join_given_strings_whith_given_separator() throws Except
         assertThat(Strings.join(",", "1", "2", "3")).isEqualTo("1,2,3");
         assertThat(Strings.join(" -> ", "aBc", "2", "DEf")).isEqualTo("aBc -> 2 -> DEf");
     }
+
+    @Test
+    public void emptyArray_should_return_empty_array()
+    {
+        assertThat(Strings.emptyArray()).isEmpty();
+    }
 }