cleanup(netx): another batch of small/simple cleanups (#789)

See https://github.com/ooni/probe/issues/2121

internal/model/logger.go

@@ -69,3 +69,12 @@ func ErrorToStringOrOK(err error) string {
 	}
 	return "ok"
 }
+
+// ValidLoggerOrDefault is a factory that either returns the logger
+// provided as argument, if not nil, or DiscardLogger.
+func ValidLoggerOrDefault(logger Logger) Logger {
+	if logger != nil {
+		return logger
+	}
+	return DiscardLogger
+}

internal/model/logger_test.go

@@ -31,3 +31,19 @@ func TestErrorToStringOrOK(t *testing.T) {
 		}
 	})
 }
+
+func TestValidLoggerOrDefault(t *testing.T) {
+	t.Run("with nil argument", func(t *testing.T) {
+		out := ValidLoggerOrDefault(nil)
+		if out != DiscardLogger {
+			t.Fatal("unexpected result")
+		}
+	})
+
+	t.Run("with non-nil argument", func(t *testing.T) {
+		in := &logDiscarder{}
+		if ValidLoggerOrDefault(in) != in {
+			t.Fatal("unexpected result")
+		}
+	})
+}