Handle case where params[:section_code] is empty

[maddiedierker]

Follow-up to #30458, as it was causing this Honeybadger error for signed in users when submitting the "join section" form with an empty section code.

More detailed repro:

1. Sign in (as any user type)
2. Visit studio.code.org/join
3. Submit the form by clicking "Go". Do not enter a section code.

EXPECTED: "Join section" form is re-rendered
ACTUAL: A 500 error "sad bee" page was displayed.

Video of fixed/expected flow:

[codecov-io]

Codecov Report

Merging #30571 into staging will increase coverage by 3.59%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           staging   #30571      +/-   ##
===========================================
+ Coverage    73.21%   76.81%   +3.59%     
===========================================
  Files         2058      680    -1378     
  Lines       112988    27798   -85190     
  Branches      3495        0    -3495     
===========================================
- Hits         82723    21352   -61371     
+ Misses       27003     6446   -20557     
+ Partials      3262        0    -3262

Flag	Coverage Δ
#integration	?
#storybook	?
#unit	?

Impacted Files	Coverage Δ
dashboard/app/controllers/followers_controller.rb	98.46% <100%> (+1.44%)	⬆️
dashboard/app/models/ability.rb	100% <0%> (ø)	⬆️
apps/src/applab/AppLabView.jsx
.../src/lib/ui/accounts/ChangeUserTypeModal.story.jsx
apps/src/applab/GoogleChart.js
...s/src/lib/ui/accounts/manageLinkedAccountsRedux.js
apps/src/lib/ui/accounts/DeleteAccountHelpers.jsx
...c/applab/designElements/ColorPickerPropertyRow.jsx
...mplates/teacherDashboard/SelectSectionDropdown.jsx
...ps/src/templates/instructions/InputOutputTable.jsx
... and 1371 more

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