Handle all messages in smartpower dimming outlet

Previously the implementation of isKnownDescription didn't cover all
possible messages that could be parsed and this caused null pointer
exceptions with certain messages.  This now handles all the
possibilities.

This resolves: https://smartthings.atlassian.net/browse/DVCSMP-2227

devicetypes/smartthings/smartpower-dimming-outlet.src/smartpower-dimming-outlet.groovy

@@ -71,7 +71,7 @@ def parse(String description) {
 
 	def event = [:]
 	def finalResult = isKnownDescription(description)
-	if (finalResult != "false") {
+	if (finalResult) {
 		log.info finalResult
 		if (finalResult.type == "update") {
 			log.info "$device updates: ${finalResult.value}"
@@ -212,13 +212,16 @@ def isKnownDescription(description) {
 		else if (descMap.cluster == "0B04" || descMap.clusterId == "0B04"){
 			isDescriptionPower(descMap)
 		}
+		else {
+			return [:]
+		}
 	}
 	else if(description?.startsWith("on/off:")) {
 		def switchValue = description?.endsWith("1") ? "on" : "off"
 		return	[type: "switch", value : switchValue]
 	}
 	else {
-		return "false"
+		return [:]
 	}
 }
 
@@ -252,7 +255,7 @@ def isDescriptionOnOff(descMap) {
 		return	[type: "switch", value : switchValue]
 	}
 	else {
-		return "false"
+		return [:]
 	}
 
 }
@@ -279,10 +282,9 @@ def isDescriptionLevel(descMap) {
 
 	if (dimmerValue != -1){
 		return	[type: "level", value : dimmerValue]
-
 	}
 	else {
-		return "false"
+		return [:]
 	}
 }
 
@@ -304,7 +306,7 @@ def isDescriptionPower(descMap) {
 		return	[type: "power", value : powerValue]
 	}
 	else {
-		return "false"
+		return [:]
 	}
 }