@bem-react/classname: lint formatter usage with null

[PavelPivkin]

The declared type of classname formatter always allows to use null as a first argument. However using formatter with null and blockMods returns only block name.

const cat = cn('Cat')

cat() // Cat

cat({ size: 'm' }) // Cat Cat_size_m
// but
cat(null, { size: 'm' }) // Cat

// mixes
cat(null, ['Dog']) // Cat Dog
// but
cat(null, { size: 'm' }, ['Dog']) // Cat

Let's fix ClassNameFormatter type https://github.com/bem/bem-react/blob/master/packages/classname/classname.ts#L21

[PavelPivkin]

PR with fix: https://github.com/bem/bem-react/pull/605