TypeError when processing enums containing "null"

[westmark]

Hi!

In V3(not sure about V2), null is a valid value for component enum properties. Attempting to run the swagger generator on a schema containing an enum null value will result in

TypeError: Cannot read property 'replace' of null

...on this line

https://github.com/drwpow/openapi-typescript/blob/14353afb246533a134247ccf720515dd441cddfe/src/v3.ts#L67

...since item can be null.

Assuming the property is properly marked as nullable, the generator should just skip the null value since | null will be prepended to the type definition anyway.

Found in 2.4.2

[drwpow]

This was fixed in #234 I believe, and should have been released in v3 of this library. Please give the new version a try, and let me know if you encounter any difficulties (and if you are still seeing the same error, please post your source schema and we can add a fix quickly!)