Exclude identities when PERCENTAGE_SPLIT trait is undefined

[emyller]

Users without specified trait appear in segment, evaluated using identity key instead.

Acceptance criteria

• Return false when trait is undefined instead of falling back to identity key

Current implementation:

flagsmith-dotnet-client/Flagsmith.Engine/Segment/Evaluator.cs

Lines 174 to 175 in 3a25c02

	else if (context.Identity?.Key != null)
	objectIds = new List<string> { segmentKey, context.Identity.Key };

See Flagsmith/flagsmith-engine#283 for context.