OpenApiOperation.Tags ignores null assignment and discards custom comparers #3115

Description

When working with OpenApiOperation, setting Tags = null does not reset the tags collection. If tags were previously assigned, assigning null is silently ignored and the old tags remain in place.

In addition, passing a HashSet with a custom equality comparer or a SortedSet gets replaced with a HashSet using OpenApiTagComparer.Instance. This differs from OpenApiDocument.Tags, which supports custom comparers, sorted sets, and resetting to null.

Repro Steps

`csharp
var operation = new OpenApiOperation
{
Tags = new HashSet { new OpenApiTagReference("tag1") }
};

// Attempt to clear or reset tags
operation.Tags = null;

// Fails: operation.Tags is still non-null and still contains tag1
Assert.Null(operation.Tags);
`

Expected Behavior

• Setting operation.Tags = null should set the property back to null.
• Custom tag comparers or sorted sets should be preserved.

Actual Behavior

• Setting operation.Tags = null has no effect because the setter only checks if (value is null) return; without assigning _tags = null;.
• Sets with custom comparers are replaced by new HashSet(value, OpenApiTagComparer.Instance).