Go: introduce defaultTypeValue to translate type to the default value of type

It will be used in KST to check that we return default values when error is expected

Author: Mingun

shared/src/main/scala/io/kaitai/struct/languages/GoCompiler.scala

@@ -485,12 +485,7 @@ class GoCompiler(typeProvider: ClassTypeProvider, config: RuntimeConfig)
   override def instanceHeader(className: List[String], instName: InstanceIdentifier, dataType: DataType, isNullable: Boolean): Unit = {
     out.puts(s"func (this *${types2class(className)}) ${publicMemberName(instName)}() (v ${kaitaiType2NativeType(dataType)}, err error) {")
     out.inc
-    translator.returnRes = Some(dataType match {
-      case _: NumericType => "0"
-      case _: BooleanType => "false"
-      case _: StrType => "\"\""
-      case _ => "nil"
-    })
+    translator.returnRes = Some(defaultTypeValue(dataType))
   }
 
   override def instanceCalculate(instName: Identifier, dataType: DataType, value: Ast.expr): Unit = {
@@ -692,6 +687,13 @@ object GoCompiler extends LanguageCompilerStatic
   def enumToStr(typeName: List[String], enumName: String): String =
     types2class(typeName) + "__" + type2class(enumName)
 
+  def defaultTypeValue(dataType: DataType): String = dataType match {
+    case _: NumericType => "0"
+    case _: BooleanType => "false"
+    case _: StrType => "\"\""
+    case _ => "nil"
+  }
+
   override def kstreamName: String = "kaitai.Stream"
   override def kstructName: String = "kaitai.Struct"
   override def ksErrorName(err: KSError): String = err match {