typescript-jquery: Update to work with Typescript strict mode

Created by: janjongboom

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

@TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @nicokoenig @topce @akehir

Description of the PR

When compiling an API generated using the typescript-jquery generator against Typescript 3.6.2 in strict mode there are a number of issues with the generated code.

• defaultExtraJQueryAjaxSettings is of type JQueryAjaxSettings | undefined but is set to null (valid in normal mode, not valid in strict mode).
• In extendObj hasOwnProperty is called on objB. But objB is not guaranteed to be an object as it inherits from unknown type T1.
• Properties in the Configuration class are marked as string, but have no initializer, thus giving false safety when consuming this class (values could be undefined too).

This PR adds some small guards around these issues.