fix(GlButton): Add conditional for icon

What does this MR do?

A tiny MVC to address: #775 (closed)

Since we wrap our button text slot in a class we add a conditional to ignore that span on Icon only buttons so we don't create extra spacing to the right.

<span v-if="!hasIconOnly" class="gl-button-text"><slot></slot></span>

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
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• Added the ~"component:*" label(s) if applicable.