[Bugfix/TradeBalance] Fix parameter name for TradeBalance and handle 'null' response from EconDB API

[asymness]

Description

• Summary of the change / bug fix:
  • The parameter being used in economy/macro for Trade Balance was incorrect (TBFR) which is specific to France. The general parameter name should be TB. I.e. TB<country_code>
  • In some cases, econdb API returns null response with 200 status code which raises an error "NoneType object is not subscriptable", the second commit fixes this issue by checking the response content before trying to parse it into a Series.
• Link # issue: [Bug]economy/macro - TBFR: Error: 'NoneType' object is not subscriptable #2152
• Screenshot of the feature or the bug before/after fix, if applicable.
  Before

After

• Relevant motivation and context: The Trade Balance indicator was not functional prior to this change in economy/macro.
• List any dependencies that are required for this change: None

How has this been tested?

• Please describe the tests that you ran to verify your changes.
  Ran macro command for various countries using TB parameter.

• Provide instructions so we can reproduce.
  Run Trade Balance indicator using TBFR parameter with any country to reproduce in economy/macro.

• Please also list any relevant details for your test configuration.
  N/A

Checklist:

• Update our Hugo documentation following these guidelines.
• Update our tests following these guidelines.
• Make sure you are following our CONTRIBUTING guidelines.
• If a feature was added make sure to add it to the corresponding scripts file.

Others

• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• My code passes all the checks pylint, flake8, black, ... To speed up development you should run pre-commit install.
• New and existing unit tests pass locally with my changes. You can test this locally using pytest tests/....

[JerBouma]

Nice catch, well done!