Fix ghost button on payment step for subscriptions

Changelog: fixed
EE: true

ee/app/assets/javascripts/vue_shared/purchase_flow/components/step.vue

@@ -124,7 +124,13 @@ export default {
           :state="isValid"
           :class="[!isValid && errorMessage ? 'gl-mb-5' : 'gl-mb-0', 'gl-mt-3']"
         />
-        <gl-button variant="success" category="primary" :disabled="!isValid" @click="nextStep">
+        <gl-button
+          v-if="nextStepButtonText"
+          variant="success"
+          category="primary"
+          :disabled="!isValid"
+          @click="nextStep"
+        >
           {{ nextStepButtonText }}
         </gl-button>
       </div>

ee/spec/frontend/vue_shared/purchase_flow/components/step_spec.js

@@ -167,7 +167,7 @@ describe('Step', () => {
         apolloProvider: mockApollo,
       });
 
-      expect(wrapper.text()).toBe('');
+      expect(wrapper.findComponent(GlButton).exists()).toBe(false);
     });
 
     it('is disabled when this step is not valid', () => {