fix: XML report was not generated by default

in the previous version, XML report was enabled by default.
so even in this v4, it should be enabled by default. Note that
the base plugin should introduce no convention, so this feature is
enabled only in the spotbugs plugin (not spotbugs-base plugin).

Author: KengoTODA

src/functionalTest/groovy/com/github/spotbugs/snom/BasePluginFunctionalTest.groovy

@@ -99,4 +99,28 @@ task spotbugsMain(type: com.github.spotbugs.snom.SpotBugsTask) {
         result.task(":classes").outcome == TaskOutcome.SUCCESS
         result.task(":spotbugsMain").outcome == TaskOutcome.SUCCESS
     }
+
+    def "SpotBugsTask does not create report by default"() {
+        setup:
+        buildFile << """
+task spotbugsMain(type: com.github.spotbugs.snom.SpotBugsTask) {
+    dependsOn 'classes'
+    classDirs = sourceSets.main.output
+}
+"""
+        when:
+        BuildResult result =
+                GradleRunner.create()
+                .withProjectDir(rootDir)
+                .withArguments(":spotbugsMain")
+                .withPluginClasspath()
+                .forwardOutput()
+                .withGradleVersion(version)
+                .build()
+
+        then:
+        result.task(":spotbugsMain").outcome == TaskOutcome.SUCCESS
+        File report = rootDir.toPath().resolve("build").resolve("reports").resolve("spotbugs").resolve("main.xml").toFile()
+        !report.isFile()
+    }
 }

src/main/groovy/com/github/spotbugs/snom/SpotBugsTask.groovy

@@ -363,7 +363,15 @@ class SpotBugsTask extends DefaultTask implements VerificationTask {
     @Optional
     @Nested
     SpotBugsReport getFirstEnabledReport() {
-        return reports.stream().filter({report -> report.enabled}).findFirst().orElse(null)
+        // use XML report by default, only when SpotBugs plugin is applied
+        boolean isSpotBugsPluingApplied = project.pluginManager.hasPlugin("com.github.spotbugs")
+
+        java.util.Optional<SpotBugsReport> report = reports.stream().filter({ report -> report.enabled}).findFirst()
+        if (isSpotBugsPluingApplied) {
+            return report.orElse(reports.create("xml"))
+        } else {
+            return report.orElse(null)
+        }
     }
 
     void setReportLevel(@Nullable String name) {