表单 select 搜索输入时，select框会往下掉

未输入内容时：

输入内容：

直接删掉 `ant-select-selection-placeholder` 节点也可以复现

猜测原因：输入内容后，`ant-select-selection-placeholder` 的节点删掉了，没有文字内容支撑，line-height 无法生效，导致 select 框往下掉

Start by reproducing the form select behavior in the two screenshots, comparing the empty search state with the state after input. Inspect the ant-select-selection-placeholder node and the select's layout when that node is removed. Done means the select box no longer shifts downward while searching, including when the placeholder node has no text.

Written by the indexing model from the issue text.