[PHP] Username checks null default value is empty string

Created by: phiamo

The php generator checks username and password for null, but default values are empty string ''

https://github.com/OpenAPITools/openapi-generator/blob/1522855915f941826879d468be0b65f798f732ca/modules/openapi-generator/src/main/resources/php/api.mustache#L510 https://github.com/OpenAPITools/openapi-generator/blob/1522855915f941826879d468be0b65f798f732ca/modules/openapi-generator/src/main/resources/php/Configuration.mustache#L60

When using e.g. an API which supports Bearer and http auth, the bearer Auth header gets replaced by: Basic Og==