MDL-67114 scssphp: php74 compatibility local fixes

Note they are already available upstream, versions 1.0.4 and up so,
in next upgrade they will be fixed.

lib/scssphp/Compiler.php

@@ -2664,11 +2664,15 @@ class Compiler
      * @param array   $value
      * @param boolean $inExp
      *
-     * @return array|\ScssPhp\ScssPhp\Node\Number
+     * @return null|array|\ScssPhp\ScssPhp\Node\Number
      */
     protected function reduce($value, $inExp = false)
     {
 
+        if (is_null($value)) {
+            return null;
+        }
+
         switch ($value[0]) {
             case Type::T_EXPRESSION:
                 list(, $op, $left, $right, $inParens) = $value;

lib/scssphp/moodle_readme.txt

@@ -7,5 +7,14 @@ Import procedure:
 
 - Copy all the files from the folder 'src' this directory.
 - Copy the license file from the project root.
+- Review the local changes defined below, if any. Reapply
+  them if needed. If already available upstream, please remove
+  them from the list.
 
 Licensed under MIT, Copyright (c) 2015 Leaf Corcoran.
+
+Currenly using 1.0.2 plus these local changes:
+
+- MDL-67114 : Added basic compatibility with php 7.4. This corresponds to
+      upstream commit https://github.com/scssphp/scssphp/commit/66675c1553b7e9d7c480d8aaedbf7c72374647cf
+      that is available in scssphp >= 1.0.4