[gromet] Support empty body for if condition

[vincentraymond-ua]

There are a few cases in CISM where there are legitimate if conditionals with empty bodies:

   if (model%options%whichdycore /= DYCORE_GLIDE) then   ! glam/glissade dycore
!!       call coordsystem_allocate(model%general%ice_grid, model%basalproc%Hwater)
!!       allocate(model%basalproc%u (ewn-1,nsn-1,model%basalproc%tnodes)); model%basalproc%u=41.0d3
!!       allocate(model%basalproc%etill (ewn-1,nsn-1,model%basalproc%tnodes));model%basalproc%etill=0.5d0
    endif

This case doesn't actually have an effect on the data overall data flow, but if the expression calls a side-effecting function, it could.