Issue #3083778 Add null checks in nodeaccess_node_grants()

Addresses a warning that was occurring when trying to access array offsets on potentially null values in the nodeaccess_node_grants() function.

Changes:

• Add a check to ensure $map_rid_gid is an array before iterating
• Add a check to ensure each $role_id exists in $map_rid_gid before accessing

These changes prevent "Trying to access array offset on value of type null" warnings and make the function more robust against unexpected data.

Closes #3083778