lib.meta.availableOn: Return false if pkg parameter is null

To fix overriding packages that checks for platform compatibility, like pipewire.

`pipewire` contains the following logic to enable support for ldac depending on library platform compatibility:
```nix
ldacbtSupport = lib.meta.availableOn stdenv.hostPlatform ldacbt
```

Which is used later in the expression to create a Meson flag:
```nix
(lib.mesonEnable "bluez5-codec-ldac" (bluezSupport && ldacbtSupport))
```

This means that attempting to build `pipewire` without `ldacbt` like:
```nix
pipewire.override {
  ldacbt = null;
}
```
will fail because the the Meson flag indicates the feature should be enabled, but the library is passed to `buildInputs` as `null`.