Check for nil sdk.Int in param validation

[karzak]

Currently, many of our parameter validations assume that sdk.Int params are not nil. If proposals get submitted that have nil parameter values, they will panic on validation.

See, for example: https://github.com/Kava-Labs/kava/pull/644/files/8493285a182aa8e29102cac883aae40f66d0352e#r488664070