Check for empty main class

This is actually what happens when the pom doesn't have this property

Author: gselzer

src/main/java/org/scijava/maven/plugin/enforcer/MainClassExistsRule.java

@@ -82,6 +82,9 @@ public void execute() throws EnforcerRuleException {
             return;
         }
         String mainClass = properties.getProperty("main-class");
+        if (mainClass.trim().isEmpty()) {
+            return;
+        }
 
         // Get the build output directory (e.g., target/classes)
         String outputDirectory = project.getBuild().getOutputDirectory();

src/test/java/org/scijava/maven/plugin/enforcer/MainClassExistsRuleTest.java

@@ -29,13 +29,6 @@
 
 package org.scijava.maven.plugin.enforcer;
 
-import static org.hamcrest.MatcherAssert.assertThat;
-import static org.hamcrest.Matchers.containsString;
-import static org.junit.Assert.assertTrue;
-
-import java.io.File;
-import java.io.IOException;
-
 import org.apache.maven.enforcer.rule.api.EnforcerLogger;
 import org.apache.maven.enforcer.rule.api.EnforcerRuleException;
 import org.apache.maven.model.Build;
@@ -44,9 +37,13 @@
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
-import org.junit.rules.ExpectedException;
 import org.junit.rules.TemporaryFolder;
 
+import java.io.File;
+import java.io.IOException;
+
+import static org.junit.Assert.assertTrue;
+
 /**
  * Tests for {@link MainClassExistsRule}.
  *
@@ -91,15 +88,23 @@ public void setUp() throws Exception {
 	}
 
 	@Test
-	public void execute_NoMainClassProperty_Passes() throws Exception {
+	public void TestNoMainClassProperty() throws Exception {
 		// No "main-class" property set
 		MainClassExistsRule rule = new MainClassExistsRuleMock(project);
         // Execute the rule and make sure it passes
 		rule.execute();
 	}
 
+    @Test
+    public void TestEmptyMainClassProperty() throws Exception {
+        // No "main-class" property set
+        MainClassExistsRule rule = new MainClassExistsRuleMock(project);
+        // Execute the rule and make sure it passes
+        rule.execute();
+    }
+
 	@Test
-	public void execute_MainClassExists_Passes() throws Exception {
+	public void TestMainClassExists() throws Exception {
 		// Set main-class property
 		project.getProperties().setProperty("main-class", "com.example.Main");
 
@@ -112,7 +117,7 @@ public void execute_MainClassExists_Passes() throws Exception {
 	}
 
 	@Test
-	public void execute_MainClassDoesNotExist_ThrowsException() throws Exception {
+	public void TestMainClassDoesNotExist() {
 		// Set main-class property...
 		project.getProperties().setProperty("main-class", "com.example.NonExistent");