fix: handle null django_id in identity overrides for local evaluation mode

[rolodato]

I'm not sure how django_id can return null here, but other SDKs do handle this possibility:

• https://github.com/Flagsmith/flagsmith-ruby-client/blob/d467610cc9ad868082aeb6ce58322dbd3cf669dc/lib/flagsmith/engine/identities/models.rb#L17
• https://github.com/Flagsmith/flagsmith-php-client/blob/7c20db24691fe5a59cc69676733df93bb543255f/src/Engine/Identities/IdentityModel.php#L23
• https://github.com/Flagsmith/flagsmith-engine/blob/902aa899ce156012e6551fe9a8499848ccafdfbf/flag_engine/features/models.py#L78

This caused an exception in the FlagsmithClient constructor similar to this:

Newtonsoft.Json.JsonSerializationException

  HResult=0x80131500

  Message=Error converting value {null} to type 'System.Int32'. Path 'identity_overrides[0].identity_features[0].django_id', line 1, position 1372.

  Source=Newtonsoft.Json

  StackTrace:

   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.EnsureType(JsonReader reader, Object value, CultureInfo culture, JsonContract contract, Type targetType)