HTML clients have Apache 2.0 license URL as default

Relates to #140

It looks like the "html" and "html2" client generators use the Apache 2.0 license URL as default if the OpenAPI document does not specify a license:
https://github.com/swagger-api/swagger-codegen-generators/blob/e34685e086e0372a205563768ee80a89ec5cae3d/src/main/java/io/swagger/codegen/v3/generators/html/StaticHtmlCodegen.java#L58
https://github.com/swagger-api/swagger-codegen-generators/blob/e34685e086e0372a205563768ee80a89ec5cae3d/src/main/java/io/swagger/codegen/v3/generators/html/StaticHtml2Codegen.java#L71

This is problematic because it is not clear in the generated HTML documentation what this license applies to, and maybe the developer who publishes the documentation for their API does not want to provide the documentation under that license.

It would be best if this default license URL was omitted, and license information was only shown in case a license is explicitly specified.

Start with src/main/java/io/swagger/codegen/v3/generators/html/StaticHtmlCodegen.java at line 58 and StaticHtml2Codegen.java at line 71, then inspect how each generator renders license information. Confirm the generated HTML omits license information when the OpenAPI document has no explicit license, while preserving explicitly supplied license details.