fix: browser env mutation.handlers is undefined (#892)
fix: browser env mutation.handlers is undefined (#892)

* Fix browser env mutation.handlers is undefined

In the latest Firefox and Google browsers, state data tree will not load for error:[mutation.handlers is undefined], because mutation handler actually passes is null (store. _mutations is empty) while pushing the handlers value as an element to the base mutations array, just add robust check.

* Fix CI lint space problem

vuejs/vue-devtools,vuejs/vue-devtools