[Collator Selection] Fix weight refund for set_candidacy_bond (#3643)

Fixes #3642 

This PR implements the weight refund of
`pallet_collator_selection::set_candidacy_bond` to account for no
iterations when the bond is decreased.

---------

Signed-off-by: georgepisaltu <george.pisaltu@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>

cumulus/pallets/collator-selection/src/lib.rs

@@ -495,7 +495,11 @@ pub mod pallet {
 				})
 				.unwrap_or_default();
 			Self::deposit_event(Event::NewCandidacyBond { bond_amount: bond });
-			Ok(Some(T::WeightInfo::set_candidacy_bond(initial_len as u32, kicked as u32)).into())
+			Ok(Some(T::WeightInfo::set_candidacy_bond(
+				bond_increased.then(|| initial_len as u32).unwrap_or_default(),
+				kicked as u32,
+			))
+			.into())
 		}
 
 		/// Register this account as a collator candidate. The account must (a) already have

prdoc/pr_3643.prdoc

@@ -0,0 +1,12 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: Fix weight refund for `pallet_collator_selection::set_candidacy_bond`
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      This PR implements the weight refund of `pallet_collator_selection::set_candidacy_bond` to
+      account for no iterations over the candidate list when the candidacy bond is decreased.
+crates:
+  - name: pallet-collator-selection