Make DefaultPluginDescriptor#addDependency usable (#398)

Thanks!

pf4j/src/main/java/org/pf4j/DefaultPluginDescriptor.java

 package org.pf4j;
 
 import java.util.ArrayList;
-import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
 
     }
 
     protected PluginDescriptor setDependencies(String dependencies) {
+        this.dependencies = new ArrayList<>();
+
         if (dependencies != null) {
             dependencies = dependencies.trim();
-            if (dependencies.isEmpty()) {
-                this.dependencies = Collections.emptyList();
-            } else {
-                this.dependencies = new ArrayList<>();
+            if (!dependencies.isEmpty()) {
                 String[] tokens = dependencies.split(",");
                 for (String dependency : tokens) {
                     dependency = dependency.trim();
                         this.dependencies.add(new PluginDependency(dependency));
                     }
                 }
-                if (this.dependencies.isEmpty()) {
-                    this.dependencies = Collections.emptyList();
-                }
             }
-        } else {
-            this.dependencies = Collections.emptyList();
         }
 
         return this;

pf4j/src/test/java/org/pf4j/DefaultPluginDescriptorTest.java

+/*
+ * Copyright (C) 2012-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pf4j;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class DefaultPluginDescriptorTest {
+
+    @Test
+    void addDependency() {
+        // Given a descriptor with empty dependencies
+        DefaultPluginDescriptor descriptor = new DefaultPluginDescriptor();
+        descriptor.setDependencies("");
+        PluginDependency newDependency = new PluginDependency("test");
+
+        // When I add a dependency
+        descriptor.addDependency(newDependency);
+
+        // Then the dependency is added
+        List<PluginDependency> expected = new ArrayList<>();
+        expected.add(newDependency);
+        assertEquals(expected, descriptor.getDependencies());
+    }
+}