Improve keeper field validation

[shaspitz]

Description

Builds off #740, with improved validation that checks for keeper fields being nil valued.

Details

Note there's a weird intricacy when calling reflect.ValueOf(x).IsZero(), where ValueOf() returns a zero valued reflect.Value if x is nil, which panics IsZero() in a way that doesn't print an easily debuggable panic message. This PR doesn't change panicking functionality, but it does ensure that a useful panic message is printed when a keeper field is nil. mustValidateFields only exists for ease of debugging anyways.

Type of change

• Refactor: Changes existing code style, naming, structure, etc.

Regression tests

Existing UTs act as regression tests

New behavior tests

n/a

Versioning Implications

• This PR will affect semantic versioning as defined for ICS

If the above box is checked, which version should be bumped?

• MAJOR: Consensus breaking changes to both the provider and consumers(s), including updates/breaking changes to IBC communication between provider and consumer(s)
• MINOR: Consensus breaking changes which affect either only the provider or only the consumer(s)
• PATCH: Non consensus breaking changes

Targeting

Please select one of the following:

• This PR is only relevant to main
• This PR is relevant to main, and should also be back-ported to ____ (ex: v1.0.0 and v1.1.0)
• This PR is only relevant to ____ (ex: v1.0.0, v1.1.0, and v1.2.0)