Empty settings are now saved as NULL.

Resolves #553

Updating Settings controller to set empty fields to `NULL` instead of an empty string. Also updating some styles for placeholders in admin and updating some seed files to initially set empty values as `NULL` instead of empty strings.

Next update, will address how these DB values are checked in the blade View templates.

@DFurnes relates to our conversation earlier regarding issues with empty strings.

CC: @barryclark