fix(Select): resolve $el undefined error

[KumJungMin]

Defect Fixes

• fix #6539
• There is an issue where this.$refs.filterInput.$el is not defined when opening the select.
• Upon investigation, I found that filterInput is only rendered when this.filter is true, so I added the this.filter condition.

this.autoFilterFocus && this.filter && focus(this.$refs.filterInput.$el); // add this.filter