Fix tab titles not rendering for plain-text tabs

titleRenderer was always set as a function reference in
buildRegistration(), so the v-if="tab.titleRenderer" guard in GTabs
was always true. TabTitleContent would then return a plain string
which Vue 2.7 can't render from a render function, producing empty
tab labels.

Only set titleRenderer when the tab actually has a title slot,
so plain-text titles fall through to the {{ tab.title }} template
interpolation.

client/src/components/BaseComponents/GTab.vue

@@ -54,17 +54,10 @@ const shouldRender = computed(() => {
     return hasBeenActive.value;
 });
 
-function titleRenderer() {
-    if (slots.title) {
-        return slots.title();
-    }
-    return undefined;
-}
-
 function buildRegistration(): TabRegistration {
     return {
         title: props.title,
-        titleRenderer,
+        titleRenderer: slots.title ? () => slots.title!() : undefined,
         disabled: props.disabled,
         id: props.id,
         buttonId: props.buttonId,