Apply type casting & remove empty kwarg for Command

[nkvuong]

Currently, all kwargs are passed into App.command as str. Empty kwarg is passed as empty string.

In this PR, we remove any empty kwarg, and apply casting based on the kwarg annotation for primitive types

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.05%. Comparing base (2b75d24) to head (cc89c55). Report is 3 commits behind head on main.

:exclamation: Current head cc89c55 differs from pull request most recent head 73c601f

Please upload reports for the commit 73c601f to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #108 +/- ## ========================================== + Coverage 78.78% 79.05% +0.27% ========================================== Files 14 14 Lines 1499 1509 +10 Branches 269 272 +3 ========================================== + Hits 1181 1193 +12 + Misses 230 229 -1 + Partials 88 87 -1 ```

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