fix #9046 disable dep collection when invoking lifecycle hooks will be useless when invoking computed in lifecycle

[ysj16]

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• Bugfix
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[ysj16]

In lifecycle.js lines316 , the method 'callHook', disable dep collection when invoking lifecycle hooks by 'pushTarget()', which means the dep collection will be useless in the lifecycle method. The principle of this is that when pushTarget(), the params--watcher is 'undefined', so in the lifecycle method--mounted, Dep.target is 'undefined', which means will not run dep collection.
But I've found a BUG in this method， when invoking computed in mounted，it will pushTarget a lazyWatcher and then popTarget, during the process, it will lose pre Dep.target, which should be 'undefined' and make it to be the renderWatcher, so will run dep collection and collect the child component‘s property by renderWatcher.

[jacekkarczmarczyk]

Shouldn't it be !=?

[posva]

Could you add a test that was failing before adding this check?

[ysj16]

@jacekkarczmarczyk distinguish undefined from null, Dep.target would be undefined in lifecycle.js line316

[ysj16]

@posva I've pushed a test