session: fix kvdb & sql session store difference

When a session has a MacaroonRecipe set, which has a nil value set for
either the `Permissions` or the `Caveats` field, the kvdb session store
would return that field as nil, while the sql prior to this commit
would return an empty array.

When the we migrate the session store from kvdb to sql, that will cause
the kvdb and the migrate session store to differ when comparing them,
which will cause the migration to fail.

This commit fixes that by ensuring that the sql session store returns
a nil value for those fields in that scenario, rather than an empty
array.

Author: ViktorT-11

session/sql_store.go

@@ -745,9 +745,12 @@ func unmarshalSession(ctx context.Context, db SQLQueries,
 
 	var macRecipe *MacaroonRecipe
 	if perms != nil || caveats != nil {
-		macRecipe = &MacaroonRecipe{
-			Permissions: unmarshalMacPerms(perms),
-			Caveats:     unmarshalMacCaveats(caveats),
+		macRecipe = &MacaroonRecipe{}
+		if perms != nil {
+			macRecipe.Permissions = unmarshalMacPerms(perms)
+		}
+		if caveats != nil {
+			macRecipe.Caveats = unmarshalMacCaveats(caveats)
 		}
 	}