[gazelle]: Handle the case where no java sources are found (#406)

Author: shs96c

java/gazelle/BUILD.bazel

@@ -42,13 +42,21 @@ go_library(
     ],
 )
 
+go_test(
+    name = "lang_test",
+    size = "small",
+    srcs = ["lang_test.go"],
+    embed = [":gazelle"],
+)
+
 go_test(
     name = "gazelle_test",
     size = "medium",
     timeout = "short",
     srcs = [
         "configure_test.go",
         "generate_test.go",
+        "lang_test.go",
         "resolve_test.go",
     ],
     embed = [":gazelle"],

java/gazelle/lang.go

@@ -264,7 +264,9 @@ func (l javaLang) Fix(c *config.Config, f *rule.File) {
 }
 
 func (l javaLang) DoneGeneratingRules() {
-	l.parser.ServerManager().Shutdown()
+	if l.parser != nil {
+		l.parser.ServerManager().Shutdown()
+	}
 	l.javaExportIndex.FinalizeIndex()
 }
 

java/gazelle/lang_test.go

@@ -0,0 +1,15 @@
+package gazelle
+
+import (
+	"testing"
+)
+
+func TestDoneGeneratingRules_NilParser(t *testing.T) {
+	lang := NewLanguage().(*javaLang)
+
+	// Simulate the case where no Java files were found (parser was never initialized)
+	lang.parser = nil
+
+	// This should not panic
+	lang.DoneGeneratingRules()
+}