Fix assigning SLC plan 1

When a claim without a student loan plan is amended the student loan
plan was being set to Plan 1, the default option in the list.
This commit includes a blank option so unrelated amendments on a claims
without a plan don't send up the Plan 1 value.

app/models/claim.rb

@@ -152,7 +152,6 @@ class Claim < ApplicationRecord
 
   validates :has_student_loan, on: [:"student-loan"], inclusion: {in: [true, false]}, allow_nil: true
   validates :student_loan_plan, inclusion: {in: STUDENT_LOAN_PLAN_OPTIONS}, allow_nil: true
-  validates :student_loan_plan, on: [:amendment], presence: {message: "Enter a valid student loan plan"}
 
   validates :bank_sort_code, on: [:amendment], presence: {message: "Enter a sort code"}
   validates :bank_account_number, on: [:amendment], presence: {message: "Enter an account number"}

app/views/admin/amendments/new.html.erb

@@ -95,7 +95,12 @@
         </div>
         <div class="govuk-grid-column-two-thirds">
           <div class="govuk-form-group">
-            <%= claim_form.select :student_loan_plan, Claim::STUDENT_LOAN_PLAN_OPTIONS.map { |option| [option.humanize, option] }, {}, { class: "govuk-select" } %>
+            <%= claim_form.select(
+              :student_loan_plan,
+              Claim::STUDENT_LOAN_PLAN_OPTIONS.map { |option| [option.humanize, option] },
+              { include_blank: true },
+              { class: "govuk-select" }
+            ) %>
           </div>
         </div>
       </div>

spec/models/claim_spec.rb

@@ -96,7 +96,6 @@
       expect(claim).not_to be_valid(:amendment)
       expect(claim.errors.map(&:message)).to contain_exactly(
         "Enter your teacher reference number",
-        "Enter a valid student loan plan",
         "Enter a sort code",
         "Enter an account number"
       )