fix: resolve citizenship bottom sheet not opening on button tap

Event(null) was swallowed by EventObserver's ?.let pattern, preventing
the CitizenshipBottomSheet from opening when tapping dashboard buttons.
Changed to Event("") with ifBlank conversion to preserve null referrer
semantics while ensuring the observer callback fires.

feature-assets/src/main/java/io/novafoundation/nova/feature_assets/presentation/balance/list/BalanceListFragment.kt

@@ -179,7 +179,7 @@ class BalanceListFragment :
         viewModel.assetViewModeModelFlow.observe { manageAssetsAdapter.setAssetViewModeModel(it) }
 
         viewModel.openCitizenshipEvent.observeEvent { referrer ->
-            CitizenshipBottomSheet.newInstance(referrer).show(childFragmentManager, "citizenship")
+            CitizenshipBottomSheet.newInstance(referrer.ifBlank { null }).show(childFragmentManager, "citizenship")
         }
 
         viewModel.shareReferralEvent.observeEvent { shareText ->

feature-assets/src/main/java/io/novafoundation/nova/feature_assets/presentation/balance/list/BalanceListViewModel.kt

@@ -414,7 +414,7 @@ class BalanceListViewModel(
     }
 
     fun basvuruClicked() = launchUnit {
-        _openCitizenshipEvent.postValue(Event(null))
+        _openCitizenshipEvent.postValue(Event(""))
     }
 
     fun shareReferralClicked() = launchUnit {