When empty string is in Text.Choices enum, than rendering generates a warning, that will be an error in many configuration

[insspb]

I have enums, where first enum database value is empty string (common 'not specified').
For such places with current configuration sphinx will generate a warning:
CRITICAL: Unexpected section title or transition.

If user like me have an option -W, than rendering will be not finished.

I will PR fix for this.