fix(components): [tree] node drag error when using custom allowDrop method

[CherishTheYouth]

• Reset dragState.value.dropNode to null when allowDrop is transfered from true to false.(For issue [Component] [tree] el-tree 拖动 #14704)

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Description

When the allowDrop function transfer from true to false, dragState.value.dropNode have not been reset, that casuse exception when treeNodeDragEnd() is triggered.

Related Issue

Fixes #14704.

Explanation of Changes

I set dragState.value.dropNode to null when dropPrev || dropInner || dropNext is false, and it will not execute the following codes in treeNodeDragEnd:

if (draggingNode && dropNode) {
....
}

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[FrontEndDog]

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[CherishTheYouth]

Thank you for your contribution!

Grateful for making time CR.