fix: do not allow stake amounts to be zero

[gluax]

Motivation

To not allow stake to be zero since it breaks behavior.

Explanation of Changes

Disallow both stake fields from being zero.

Testing

Two new tests.

Related PRs and Issues

N/A

[github-actions]

Package	Line Rate	Complexity	Health
contract.src.msgs.data_requests.state	98%	0	✔
contract.src.msgs.data_requests	97%	0	✔
contract.src	86%	0	✔
contract.src.msgs.data_requests.sudo	92%	0	✔
contract.src.msgs.owner.execute	96%	0	✔
contract.src.msgs.owner	100%	0	✔
contract.src.msgs.staking.execute	97%	0	✔
contract.src.msgs.staking	96%	0	✔
contract.src.msgs.staking.state	96%	0	✔
contract.src.msgs	93%	0	✔
contract.src.msgs.data_requests.execute	100%	0	✔
Summary	96% (3970 / 4121)	0	✔