Deal with blank company_nos correctly. (#56)

Instead of just asking the user to enter a number, the app was attempting to turn the blank input into a valid number by adding 0s. This resulted in a company_no of 00000000. We should just give them a useful validation error instead.

app/forms/registration_number_form.rb

@@ -12,7 +12,7 @@ def submit(params)
     # Assign the params for validation and pass them to the BaseForm method for updating
     # If param isn't set, use a blank string instead to avoid errors with the validator
     self.company_no = params[:company_no] || ""
-    self.company_no = process_company_no(company_no)
+    self.company_no = process_company_no(company_no) if company_no.present?
     attributes = { company_no: company_no }
 
     super(attributes, params[:reg_identifier])

config/locales/forms/registration_number_forms/en.yml

@@ -18,6 +18,7 @@ en:
               inactive: "Your company must be registered as an active company"
               not_found: "Companies House couldn't find a company with this number"
               error: "There was an error with Companies House"
+              blank: "Enter a company registration number"
             reg_identifier:
               invalid_format: "The registration ID is not in a valid format"
               no_registration: "There is no registration matching this ID"