txscript: Optimize typeOfScript for null data scripts

This continues the process of converting the typeOfScript function to
use a combination of raw script analysize and the tokenizer instead of
parsed opcode, with the intent of significanty optimizing the function.

In particular, it converts the detection of null data scripts to use raw
script analysis.

txscript/standard.go

@@ -450,6 +450,8 @@ func typeOfScript(scriptVersion uint16, script []byte) ScriptClass {
 		return ScriptHashTy
 	case isMultisigScript(scriptVersion, script):
 		return MultiSigTy
+	case isNullDataScript(scriptVersion, script):
+		return NullDataTy
 	}
 
 	pops, err := parseScript(script)
@@ -461,9 +463,8 @@ func typeOfScript(scriptVersion uint16, script []byte) ScriptClass {
 		return WitnessV0PubKeyHashTy
 	} else if isWitnessScriptHash(pops) {
 		return WitnessV0ScriptHashTy
-	} else if isNullData(pops) {
-		return NullDataTy
 	}
+
 	return NonStandardTy
 }