Bazaar: Added 'isBazaarAvailable' helper

The EntityBazaarInfoCard shows the bazaar data for components that
have a bazaar item linked. The isBazaarAvailable helper makes it
possible to only include the EntityBazaarInfoCard for components
that actually have a bazaar project linked.

Closes #15013

Signed-off-by: Niklas Aronsson <niklasar@axis.com>

plugins/bazaar/README.md

@@ -49,17 +49,20 @@ Add a **Bazaar icon** to the Sidebar to easily access the Bazaar. In `packages/a
 Add a **Bazaar card** to the overview tab on the `packages/app/src/components/catalog/EntityPage.tsx` add:
 
 ```diff
-+ import { EntityBazaarInfoCard } from '@backstage/plugin-bazaar';
++ import { EntityBazaarInfoCard, isBazaarAvailable } from '@backstage/plugin-bazaar';
 
 const overviewContent = (
 
     <Grid item md={8} xs={12}>
       <EntityAboutCard variant="gridItem" />
     </Grid>
-
-+   <Grid item sm={6}>
-+     <EntityBazaarInfoCard />
-+   </Grid>
++   <EntitySwitch>
++     <EntitySwitch.Case if={isBazaarAvailable}>
++       <Grid item sm={6}>
++         <EntityBazaarInfoCard />
++       </Grid>
++     </EntitySwitch.Case>
++    </EntitySwitch>
 
     {/* ...other entity-cards */}
 ```