Fix mtx select appeareance and vertical alignment when not using placeholder.

[snow3461]

Hi there, I hope its ok for me to make this PR.

This PR is a proposal to fix the appearance of the mtx-select component, relative to vertical aligment, and arrow positioning.

Vertical alignment was different whether you set the placeholder property or not, unless you used the following css rule:

.mat-form-field {
vertical-align : middle;
}

It doesn't fix the problem for outline style mat-form-field, but this problem lies in angular material itself (the same behavior can be observed with mat-select )

[nzbin]

Do you test the styles?

[nzbin]

Many thanks for your PR. The mtx-select is just a wrapper for ng-select, there has many styles are not guaranteed to be exactly same with Material. I think you can modify it by your own needs.