Return an empty string in callback function
Return an empty string in callback function

By returning an empty string in latex#latexmk#callback function,
gvim does not open an error window with value "0" after each
compilation.

kmarius/vimtex,lervag/vimtex,Aster89/vimtex,kmarius/vimtex,lervag/vimtex,Aster89/vimtex