Excludes falsy values from request options

[sPaCeMoNk3yIam]

https://github.com/yarax/swagger-to-graphql/blob/master/src/getRequestOptions.ts#L54
A value should be allowed to be null, false or 0 and if set not throw an error.
Here the only valid check and throw should be for undefined.

https://github.com/yarax/swagger-to-graphql/blob/master/src/getRequestOptions.ts#L58
A value should be allowed to be null, false or 0 and be passed as param.
Here only undefined should not be passed.