Fix panic on metricbeat test modules (elastic#18797)

Since metricbeat light modules support processors (elastic#15923), module
initialization requires a publisher in the beat so modules can attach
their processors. `metricbeat test modules` is not initializing as
normal metricbeat commands, and it is not initializing any output or
publisher pipeline, so metricbeat panics when trying to initialize
modules with the new method.

This change adds a dummy publisher for this case, and fixes also a
condition that was adding a `nil` module option, causing additional
panics. A test that reproduced the issues is also added.

(cherry picked from commit 25b8bf1)

CHANGELOG.next.asciidoc

@@ -184,6 +184,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Fix storage metricset to allow config without region/zone. {issue}17623[17623] {pull}17624[17624]
 - Fix overflow on Prometheus rates when new buckets are added on the go. {pull}17753[17753]
 - Fix tags_filter for cloudwatch metricset in aws. {pull}18524[18524]
+- Fix panic on `metricbeat test modules` when modules are configured in `metricbeat.modules`. {issue}18789[18789] {pull}18797[18797]
 - Add missing network.sent_packets_count metric into compute metricset in googlecloud module. {pull}18802[18802]
 
 *Packetbeat*

metricbeat/cmd/test/modules.go

@@ -25,6 +25,7 @@ import (
 
 	"github.com/elastic/beats/v7/libbeat/beat"
 	"github.com/elastic/beats/v7/libbeat/cmd/instance"
+	"github.com/elastic/beats/v7/libbeat/publisher/pipeline"
 	"github.com/elastic/beats/v7/libbeat/testing"
 	"github.com/elastic/beats/v7/metricbeat/beater"
 )
@@ -49,6 +50,8 @@ func GenTestModulesCmd(name, beatVersion string, create beat.Creator) *cobra.Com
 				os.Exit(1)
 			}
 
+			// A publisher is needed for modules that add their own pipelines
+			b.Beat.Publisher = newPublisher()
 			mb, err := create(&b.Beat, b.Beat.BeatConfig)
 			if err != nil {
 				fmt.Fprintf(os.Stderr, "Error initializing metricbeat: %s\n", err)
@@ -78,3 +81,17 @@ func GenTestModulesCmd(name, beatVersion string, create beat.Creator) *cobra.Com
 		},
 	}
 }
+
+type publisher struct {
+	beat.PipelineConnector
+}
+
+// newPublisher returns a functional publisher that does nothing.
+func newPublisher() *publisher {
+	return &publisher{pipeline.NewNilPipeline()}
+}
+
+// SetACKHandler is a dummy implementation of the ack handler for the test publisher.
+func (*publisher) SetACKHandler(beat.PipelineACKHandler) error {
+	return nil
+}

metricbeat/mb/module/configuration.go

@@ -30,7 +30,7 @@ func ConfiguredModules(modulesData []*common.Config, configModulesData *common.C
 	var modules []*Wrapper
 
 	for _, moduleCfg := range modulesData {
-		module, err := NewWrapper(moduleCfg, mb.Registry, nil)
+		module, err := NewWrapper(moduleCfg, mb.Registry, moduleOptions...)
 		if err != nil {
 			return nil, err
 		}

metricbeat/tests/system/test_cmd.py

@@ -124,6 +124,21 @@ def test_modules_test(self):
         assert self.log_contains("cpu...OK")
         assert self.log_contains("memory...OK")
 
+    def test_modules_test_with_module_in_main_config(self):
+        self.render_config_template(reload=False, modules=[{
+            "name": "system",
+            "metricsets": ["cpu", "memory"],
+            "period": "10s",
+        }])
+
+        exit_code = self.run_beat(
+            logging_args=None,
+            extra_args=["test", "modules"])
+
+        assert exit_code == 0
+        assert self.log_contains("cpu...OK")
+        assert self.log_contains("memory...OK")
+
     def test_modules_test_error(self):
         """
         Test test modules command with an error result