php: allow php as empty

Since commit c4ffcaa2[php: split out phpdbg into a separate package],
package php is empty, we might met error:
nothing provides php needed by php-cli-7.4.9-r0.corei7_64

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>
(cherry picked from commit 9be6b4f5a2)
Signed-off-by: Armin Kuster <akuster808@gmail.com>

meta-oe/recipes-devtools/php/php_7.4.9.bb

@@ -213,6 +213,8 @@ RDEPENDS_${PN}-cli = "${PN}"
 RDEPENDS_${PN}-modphp = "${PN} apache2"
 RDEPENDS_${PN}-opcache = "${PN}"
 
+ALLOW_EMPTY_${PN} = "1"
+
 INITSCRIPT_PACKAGES = "${PN}-fpm"
 inherit update-rc.d