TermsOfService null produces run-time error with aspnetcore server stub generator

Description

When using the aspnetcore server stub generator, this generation command results with broken output if the user fails to provide a value for the termsOfService additional property.

Take this generation CLI command, which is about as basic a getting-started experience as a user would expect to make, with as few parameters as possible.

# generates aspnetcore server stub, outputs locally to ./out/petstore/server
./run-in-docker.sh generate -i modules/swagger-codegen/src/test/resources/2_0/petstore.yaml \
-l aspnetcore \
-o /gen/out/petstore/server \
--additional-properties packageName=PetStore.Api

This results in C# code that looks like this:

TermsOfService = new Uri("")

At run-time, this causes an error, as the Uri shouldn't be empty, and when empty, throws. I've prepared a pull request to accommodate this issue's resolution, which I'll submit along with the issue for consideration.

Thanks! I'd like to add an asp.net core 3.1 generator, too, so I may reach out if I run into any issues or have questions - anyone specific I should mention?

Start with the generation command in run-in-docker.sh using modules/swagger-codegen/src/test/resources/2_0/petstore.yaml and the aspnetcore generator, then inspect the generated C# around TermsOfService. Reproduce the empty-value case and verify that the generated output no longer causes a runtime Uri error when termsOfService is omitted.