Add tests to SpecialValueParameterResolverExtensionTest

src/test/java/org/jenkinsci/plugins/badge/extensions/SpecialValueParameterResolverExtensionTest.java

@@ -2,6 +2,7 @@
 
 import static org.hamcrest.CoreMatchers.is;
 import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.nullValue;
 import static org.mockito.Mockito.when;
 
 import hudson.model.Job;
@@ -73,4 +74,31 @@ void testMultipleParameters() {
                 extension.resolve(mockRun, "buildId buildNumber duration startTime displayName description");
         assertThat(actualParameter, is("1234 1234 23:35 23:35 display name run description"));
     }
+
+    @Test
+    void resolveEmptyStringParameter() {
+        String actualParameter = extension.resolve(mockRun, "");
+        assertThat(actualParameter, is(""));
+    }
+
+    @Test
+    void resolveWithNullParameter() {
+        String actualParameter = extension.resolve(mockRun, null);
+        assertThat(actualParameter, is(nullValue()));
+    }
+
+    @Test
+    void unknownParameterShouldNotBeResolved() {
+        String actualParameter = extension.resolve(mockRun, "unknown");
+        assertThat(actualParameter, is("unknown"));
+    }
+
+    @Test
+    void testResolveWhenJobHasNoLastBuild() {
+        Job<?, ?> job = Mockito.mock(Job.class);
+        when(job.getLastBuild()).thenReturn(null);
+
+        String parameter = "${buildId}${buildNumber}";
+        assertThat(extension.resolve(job, parameter), is(parameter));
+    }
 }