[csharp-netcore]Model's FromJson method check for jsonstring is not nullOrEmpty before deserializing

Created by: Ghufz

The model which has oneOf and anyOf relationship, in FromJson method jsonstring must be checked for null Or Empty before de-serializing

When the server return any exception related to connectivity , the model which contains oneOf or AnyOf throws exception for empty jsonstring. "Error reading JObject from JsonReader. Path '', line 0, position 0."

the if condition is checking jsonstring for null only.

if (jsonstring == null){
return ModelObject.
}

whereas when the server is not reachable the jsonstring become empty string which throws exception "Error reading JObject from JsonReader. Path '', line 0, position 0." which overrides the actual exception of server connectivity.

added additional check like

if (string.IsNullOrEmpty(jsonstring)){

}

to get the meaningful server side error. exception after fix "No such host is known. (xyz.com:443)"

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master, 5.3.x, 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request. @wing328