Add available annotation to Candidate extension

Author: andrewheard

FirebaseAI/Sources/Types/Internal/Errors/EmptyContentError.swift

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+@available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
 extension Candidate {
   struct EmptyContentError: Error {
     let localizedDescription = "Invalid Candidate: empty content and no finish reason"