Fix/Config variable set to None after CTRL+X

[mhrvth]

Before submitting this PR, please make sure:

• Your code builds clean without any errors or warnings
• You are using approved title ("feat/", "fix/", "docs/", "refactor/")

A description of the changes proposed in the pull request:

• Fixes a bug where config variable is set to None when an invalid value is provided and CTRL+X is entered afterwards

Tests performed by the developer:

• Tried to reproduce the error with target_asset_amount in TWAP
• Ran unit tests

Tips for QA testing:

• Try to reproduce the problem with different kinds of variables. The result should always be the previously set value, not None.

[dennisocana]

Proposal created: https://snapshot.org/#/hbot-prp.eth/proposal/0xe8784317fd70bf715571e16714aa19a8b1fcb01698addee1be5efc19dd152033

[rapcmia]

Test performed:

• Setup all hummingbot strategies available
• Select random parameter, example order_amount
• Enter invalid input and then ctrl+x to cancel, previous value still available
• Tested on source and docker build