Fix compiler warnings in example projects (apple#506)

### Motivation

A recent example project was added that had a compiler warning which
wasn't caught by CI. We can have our example code free of warnings and
ensure they remain so by compiling them with `-warnings-as-errors` in
CI.

On further investigation, we cannot enable `-warnings-as-errors` in the
examples Ci pipeline because we're reusing the shared build/cache
directories which produces its own warnings for left over unused
modules.

### Modifications

- Fix compiler warnings in example.
- ~Build with `-warnings-as-errors` in examples CI pipeline.~

### Result

Example projects compile without warnings.

### Test Plan

~CI updated.~

Author: simonjbeaumont

Examples/event-streams-server-example/Sources/EventStreamsServer/GreetingStream.swift

@@ -25,7 +25,7 @@ final class StreamStorage: @unchecked Sendable {
     private func finishedStream(id: String) {
         lock.lock()
         defer { lock.unlock() }
-        guard let task = locked_streams[id] else { return }
+        guard locked_streams[id] != nil else { return }
         locked_streams.removeValue(forKey: id)
         print("Finished stream \(id)")
     }