You can also write with this workaround using isNULL(...) function:

SELECT e.equip_id, m.freq FROM equip e
INNER JOIN maint_equip me ON isNULL(me.mfg_id,-1) = isNULL(e.mfg_id,-1) and ...
INNER JOIN maint m ON m.maint_id = me.main_id;