chromebox_for_meetings: make PolicyInfo nullable

When a client requests PolicyInfo, but Chrome's service hasn't collected
any policy info from the device yet, a nullptr is returned. This
currently causes an unexpected null validation error in Mojo on the
client. Fix this by marking PolicyInfo optional and checking for nullptr
in the client before using the returned PolicyInfoPtr.

Bug: b:175792552
Change-Id: I409a83364434201c55039078ad9d183bd160cb56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2597905
Commit-Queue: Kyle Williams <kdgwill@chromium.org>
Reviewed-by: Kyle Williams <kdgwill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839480}

chromeos/services/chromebox_for_meetings/public/mojom/meet_devices_info.mojom

@@ -14,7 +14,7 @@ interface MeetDevicesInfo {
 
   // Request device policy information
   // Note: will return nullptr on failure to query policy information
-  GetPolicyInfo@1()=>(PolicyInfo info);
+  GetPolicyInfo@1()=>(PolicyInfo? info);
 
   // Request device system information
   GetSysInfo@2()=>(SysInfo info);