Output and Response Caching treat absent and empty query/header values as the same cache key

When Output Caching or Response Caching varies by a configured query key or header, its cache key does not distinguish between the key/header being absent and the key/header being present with an empty value (such as `?mode=`).

The application layer does distinguish these states through `Request.Query.ContainsKey("mode")` and `Request.Headers.ContainsKey("X-Mode")`.

Example:
```csharp
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddOutputCache();
builder.Services.AddResponseCaching();
var app = builder.Build();
app.UseResponseCaching();
app.UseOutputCache();
var counter = 0;
app.MapGet("/output/query", (HttpContext context) =>
{
return new
{
Present = context.Request.Query.ContainsKey("mode"),
Value = context.Request.Query["mode"].ToString(),
Counter = Interlocked.Increment(ref counter),
GeneratedAt = DateTimeOffset.UtcNow
};
})
.CacheOutput(policy => policy
.Expire(TimeSpan.FromSeconds(20))
.SetVaryByQuery("mode"));
app.Run();
```

```text
curl "http://localhost:5000/output/query"
curl "http://localhost:5000/output/query?mode="
```

Start by tracing how Output Caching and Response Caching construct keys for configured query and header variation. Use the provided /output/query example and curl requests as the reproduction, then add regression coverage showing that an absent value and an explicitly empty value produce distinct cache entries.

Written by the indexing model from the issue text.