Improve use of buildClassName fn

[Yeti-or]

Just a little improvement.
buildClassName has a lot of checks before it finally gets to case with only block variable.
It seems that we can omit buildClassName for such cases.

    buildClassName : function(block, elem, modName, modVal) {
        if(isSimple(modName)) {
            if(!isSimple(modVal)) {
                modVal = modName;
                modName = elem;
                elem = undef;
            }
        } else if(typeof modName !== 'undefined') {
            modName = undef;
        } else if(elem && typeof elem !== 'string') {
            elem = undef;
        }

        if(!(elem || modName)) { // optimization for simple case
            return block;
        }
        ...

[coveralls]

Coverage remained the same at 62.461% when pulling 5e00072 on yeti-or.imprv-findEnt into 5ee9475 on v4.

[Yeti-or]

@tadatuta you don't like such change?

[tadatuta]

@Yeti-or thank you!