Allow conditional rendering inside `SidenavHeader`

Summary

Actually this pattern is not possible:

<Sidenav compact>
  <SidenavHeader label="Conditional render">
     <SidenavItem label="classic item" />
     {isDev && <SidenavItem label="not possible yet" />}
  </SidenavHeader>
</Sidenav>

When isDev === false:

[fabien0102]

This should probably works ^^ It’s actually not a big or complex issue 😉 I was just totally inside a rush when I wrote this issue, so it’s more to avoid to forget 😉

[nick-barth]

I concur, while without proper context, it’s hard for us to say this is the definitive answer, but from what we currently know, a null check in SidenavHeader would allow us to conditionally render using &&.