[Suggestion] Simplify several checks for empty/null strings.

[andrueastman]

Several instances of the code in the 2.0.0 branch can be simplified from

if (string.IsNullOrEmpty(referenceId))
{
    Utils.CheckArgumentNullOrEmpty(referenceId);
}

to this

Utils.CheckArgumentNullOrEmpty(referenceId);

as CheckArgumentNullOrEmpty checks if the string is null or empty again before throwing.

Some examples can be found below

• OpenAPI.NET/src/Microsoft.OpenApi/Models/References/OpenApiCallbackReference.cs

  Line 41 in 264226b

  if (string.IsNullOrEmpty(referenceId))

• OpenAPI.NET/src/Microsoft.OpenApi/Models/References/OpenApiLinkReference.cs

  Line 43 in 264226b

  if (string.IsNullOrEmpty(referenceId))

• OpenAPI.NET/src/Microsoft.OpenApi/Models/References/OpenApiRequestBodyReference.cs

  Line 43 in 264226b

  if (string.IsNullOrEmpty(referenceId))

• OpenAPI.NET/src/Microsoft.OpenApi/Models/References/OpenApiResponseReference.cs

  Line 43 in 264226b

  if (string.IsNullOrEmpty(referenceId))

• OpenAPI.NET/src/Microsoft.OpenApi/Models/References/OpenApiParameterReference.cs

  Line 47 in 264226b

  if (string.IsNullOrEmpty(referenceId))