Fix Nullable of return value

Author: jjohannes

samples/basic/lib/src/test/java/module-info.java

@@ -1,4 +1,4 @@
-open module org.my.app.test {
+open module org.my.lib.test {
     requires org.my.lib;
     requires org.junit.jupiter.api;
 }

src/main/java/de/jjohannes/gradle/moduledependencies/JavaModuleDependenciesExtension.java

@@ -10,6 +10,7 @@
 import javax.annotation.Nullable;
 import java.io.IOException;
 import java.io.InputStream;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Properties;
@@ -51,12 +52,11 @@ public String ga(String moduleName) {
      *
      *  If no version is defined, the version entry will be missing.
      */
-    @Nullable
     public Map<String, Object> gav(String moduleName) {
         Map<String, Object> gav = new HashMap<>();
         String ga = ga(moduleName);
         if (ga == null) {
-            return null;
+            return Collections.emptyMap();
         }
 
         VersionConstraint version = null;

src/main/java/de/jjohannes/gradle/moduledependencies/JavaModuleDependenciesPlugin.java

@@ -104,7 +104,7 @@ private void declareDependency(String moduleName, @Nullable String ownModuleName
         Map<String, Object> gav = javaModuleDependencies.gav(moduleName);
         String projectName = ownModuleNamesPrefix == null ? null : moduleName.startsWith(ownModuleNamesPrefix + ".") ? moduleName.substring(ownModuleNamesPrefix.length() + 1) : null;
 
-        if (gav != null) {
+        if (!gav.isEmpty()) {
             project.getDependencies().add(configuration.getName(), gav);
             if (!gav.containsKey(GAV.VERSION)) {
                 warnVersionMissing(moduleName, gav, moduleInfoFile, project, javaModuleDependencies);