Gravar-mail: A new example of intra­molecular C—H⋯Ni anagostic inter­actions: synthesis, crystal structure and Hirshfeld analysis of cis-bis­[4-methyl-2-(1,2,3,4-tetra­hydro­naphthalen-1-yl­idene)hydrazinecarbo­thio­amidato-κ(2) N (1),S]nickel(II) di­methyl­formamide monosolvate