No, there is no study which proves a relation between adding molasses to the soil and the buds getting sweeter. This is because molasses are used only for the purpose of enrichment of the soil and at no point are they getting absorbed into the plant that you are growing. Most people who have grown cannabis while using molasses to improve the quality of their soil have noticed no significant change in the taste of their bud. This is mostly an urban legend originating from the fact that molasses are a sugar-rich substance.