Fix some code quality and bug-risk issues

[pnijhara]

• Remove length check in favour of truthiness of the object
• Fix dangerous default argument
• Remove unnecessary f-string

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Find the other issues found here - https://deepsource.io/gh/pnijhara/gs-quant/issues/?category=all
This PR also adds .deepsource.toml configuration file to run DeepSource analysis on the repo with. Upon enabling DeepSource, the analysis will run on every PR and commit to detect 560+ types of issues in the changes — including bug risks, anti-patterns, security vulnerabilities, etc.

To enable DeepSource analysis after merging this PR, please follow these steps:

1. Signup on DeepSource with your GitHub account and grant access to this repo.
2. Activate analysis on this repo here.

You can also look at the docs for more details. Do let me know if I can be of any help!