Revert "syntax: Remove empty rules in regions"

This reverts commit a9b513a28a.

runtime/syntax/xml.yaml

@@ -8,10 +8,12 @@ rules:
     - preproc:
         start: "<!DOCTYPE"
         end: "[/]?>"
+        rules: []
 
     - comment:
         start: "<!--"
         end: "-->"
+        rules: []
 
     - symbol.tag:
         start: "<\\??"
@@ -20,6 +22,7 @@ rules:
             - identifier:
                 start: " "
                 end: "="
+                rules: []
             - constant.string:
                 start: "\""
                 end: "\""