Add better handling for edge cases

[shuvamkr]

What does this PR do?

Have made the following changes:

• isEmpty() : Removed unnecessary checks. handled the case for Set and Map.
• getSelectedTabIndex(): Added type checks for input and also optimised the approach with findIndex()
• downloadFile(): Handled the case for invalid download method provided

What packages have been affected by this PR?

• Ella

Types of changes

What types of changes does your code introduce to this project?

Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)

• New feature (non-breaking change which adds functionality)

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Package version increase in any of the packages?

Checklist before merging

Put an x in the boxes that apply

• These changes have been thoroughly tested.

• Changes need to be immediately published on npm.