Fixes issue #1636

Some frameworks (e.g. django rest framework, I guess) generate following schema:

openapi: 3.0.3
info:
    version: 1.0.0
components:
    schemas:
        BlankEnum:
            enum:
                - ''

right now this library generates invalid typescript:

export enum BlankEnum {
     = '',
}

This happens in getEnum functions that don't check for invalid input (getEnum([''])) in our case.

I decided to give such enum values BLANK name so that generated files are correct

export enum BlankEnum {
    BLANK = '',
}

Not sure if it would be a better idea to just omit these values (I mean, the name says BlankEnum) but everything's better than broken syntax