[SQL]Update MultiInstanceRelation.scala

[baishuo]

I think if multiAppearance is empty,we can return plan directly

[AmplabJenkins]

Can one of the admins verify this patch?

[baishuo]

Can one of the admins verify this patch?:)

[marmbrus]

I think this patch is correct, but I'm not convinced that we need to make these kind of optimizations at the expense of code complexity / length. I'd imagine the performance benefit of this change is negligible and AFAICT it produces the same result as the original code.

[marmbrus]

Also, for future reference there are several stylistic issues:

• Space between if and (
• Spaces } else {
• Space between ) and {

[baishuo]

thank you @marmbrus , close this PR