Fix C# codegen of optional fields

Author: qed-

main/java/uk/co/real_logic/sbe/generation/csharp/CSharpGenerator.java

@@ -704,9 +704,9 @@ private CharSequence generateFieldNotPresentCondition(final int sinceVersion, fi
  }
 
  return String.format(
- indent + " if (actingVersion < %1$d) return %2$s;\n\n",
+ indent + " if (_actingVersion < %1$d) return %2$s;\n\n",
  Integer.valueOf(sinceVersion),
- sinceVersion > 0 ? generateLiteral(encoding.primitiveType(), encoding.nullValue().toString()) : "(byte)0"
+ sinceVersion > 0 ? generateLiteral(encoding.primitiveType(), encoding.applicableNullValue().toString()) : "(byte)0"
  );
  }
 
@@ -1096,7 +1096,7 @@ private CharSequence generateEnumFieldNotPresentCondition(final int sinceVersion
  }
 
  return String.format(
- indent + " if (actingVersion_ < %d) return %s.NULL_VALUE;\n\n",
+ indent + " if (_actingVersion < %d) return %s.NULL_VALUE;\n\n",
  Integer.valueOf(sinceVersion),
  enumName
  );