Empty \N{} in regex pattern should force /d to /u

\N{} is for Unicode names, even if the name is actually omitted.
(Accepting an empty name is, I believe, an accident, and now is
supported only for backwards compatibility.)

regcomp.c

@@ -11174,6 +11174,8 @@ S_grok_bslash_N(pTHX_ RExC_state_t *pRExC_state, regnode** node_p,
 	vFAIL("\\N{NAME} must be resolved by the lexer");
     }
 
+    RExC_uni_semantics = 1; /* Unicode named chars imply Unicode semantics */
+
     if (endbrace == RExC_parse) {   /* empty: \N{} */
 	if (node_p) {
 	    *node_p = reg_node(pRExC_state,NOTHING);
@@ -11185,7 +11187,6 @@ S_grok_bslash_N(pTHX_ RExC_state_t *pRExC_state, regnode** node_p,
         return 0;
     }
 
-    RExC_uni_semantics = 1; /* Unicode named chars imply Unicode semantics */
     RExC_parse += 2;	/* Skip past the 'U+' */
 
     endchar = RExC_parse + strcspn(RExC_parse, ".}");

t/re/re_tests

@@ -1433,6 +1433,7 @@ foo(\h)bar	foo\tbar	y	$1	\t
 # Verify that \N{U+...} forces Unicode rules
 /\N{U+41}\x{c1}/i	a\x{e1}	y	$&	a\x{e1}
 /[\N{U+41}\x{c1}]/i	\x{e1}	y	$&	\x{e1}
+/\N{}\xe4/i	\xc4	y	$&	\xc4		# Empty \N{} should change /d to /u
 
 [\s][\S]	\x{a0}\x{a0}	n	-	-	# Unicode complements should not match same character