fix(Button): display variant only when set (#5458)

This bugfix aligns the `<button>` variant class implementation with
Alert/Badge/Navbar -- only outputing a `prefix-variant` formatted class
when a variant is specified and avoiding the situation of outputting
`.btn-null` or `.btn-` when the variant is null or an empty string
respectively

Split from #5456

src/Button.tsx

@@ -106,7 +106,7 @@ const Button: Button = React.forwardRef(
       className,
       prefix,
       active && 'active',
-      `${prefix}-${variant}`,
+      variant && `${prefix}-${variant}`,
       block && `${prefix}-block`,
       size && `${prefix}-${size}`,
     );

test/ButtonSpec.js

@@ -110,6 +110,24 @@ describe('<Button>', () => {
     mount(<Button>Title</Button>).assertSingle(`.btn-primary`);
   });
 
+  it('Should remove default variant', () => {
+    mount(<Button variant={null}>Title</Button>)
+      .find(`.btn-primary`)
+      .should.have.length(0);
+  });
+
+  it('Should not output null variant', () => {
+    mount(<Button variant="">Title</Button>)
+      .find(`.btn-null`)
+      .should.have.length(0);
+  });
+
+  it('Should not output empty variant', () => {
+    mount(<Button variant="">Title</Button>)
+      .find(`.btn-`)
+      .should.have.length(0);
+  });
+
   it('Should be active', () => {
     mount(<Button active>Title</Button>).assertSingle(`.active`);
   });