WITH_META_MODE: Don't try showing command if .ERROR_META_FILE is empty.

This was sed'ing on stdin for failing .PHONY targets.

Reported by:	Mark Millard
X-MFC-With:	r319862
MFC after:	1 week
Sponsored by:	Dell EMC Isilon

share/mk/local.sys.mk

@@ -13,7 +13,8 @@ MAKE_PRINT_VAR_ON_ERROR += \
 	.MAKE.MODE
 .endif
 
-_ERROR_CMD=${sed -n '/^CMD/s,^CMD ,,p' ${.ERROR_META_FILE}:L:sh}
+_ERROR_CMD_EXEC=	${sed -n '/^CMD/s,^CMD ,,p' ${.ERROR_META_FILE}:L:sh}
+_ERROR_CMD=		${!empty(.ERROR_META_FILE):?${_ERROR_CMD_EXEC}:.PHONY}
 MAKE_PRINT_VAR_ON_ERROR+= \
 	_ERROR_CMD \
 	.CURDIR \