The Legislature shall never grant any extra compensation to any public officer, employee, agent or contractor after the services shall have been rendered or the contract entered into, nor authorize the payment of any claims or part thereof created against the state, under any agreement or contract made without express authority of law, and all such unauthorized agreements or contracts shall be null and void; nor shall the compensation of any public officer be increased or diminished during his term of office: provided, however, that the Legislature may make appropriations for expenditures incurred in suppressing insurrection or repelling invasion