be: Simply use 'undef' instead of additional special handling for empty strings.

ir/be/ia32/ia32_spec.pl

@@ -805,7 +805,7 @@ Cmc => {
 	out_reqs => [ "flags" ],
 	fixed    => "x86_condition_code_t condition_code = x86_cc_carry;\n"
 	           ."\tx86_insn_size_t const size = X86_SIZE_32;",
-	attr     => "",
+	attr     => undef,
 	emit     => "cmc",
 	encode   => "ia32_enc_simple(0xF5)",
 	latency  => 1,
@@ -946,7 +946,7 @@ IJmp => {
 Const => {
 	template  => $valueop,
 	emit      => "movl %I, %D0",
-	fixed     => "",
+	fixed     => undef,
 	attr      => "const x86_imm32_t *imm",
 	fixed     => "x86_insn_size_t const size = X86_SIZE_32;",
 	attr_type => "ia32_immediate_attr_t",

ir/be/scripts/generate_new_opcodes.pl

@@ -182,9 +182,7 @@ sub create_constructor
 	}
 	# we have additional attribute arguments
 	if (defined(my $attr = $n->{attr})) {
-		if ($attr ne "") {
-			$complete_args .= ", $attr";
-		}
+		$complete_args .= ", $attr";
 	}
 
 	my $suffix = $name ne "" ? "_$name" : "";