fix: skip student loan uprating if column doesn't exist (#1500)

During initial dataset creation in policyengine-uk-data, the
student_loan_plan column is added by imputation after the base
FRS dataset is created. The uprating code runs before imputation,
so we need to gracefully skip if the column doesn't exist yet.

Author: nwoodruff-co

policyengine_uk/data/economic_assumptions.py

@@ -245,6 +245,11 @@ def uprate_student_loan_plans(
     For (2) and (3), we use highest_education == TERTIARY as the signal
     for who is a graduate, then apply a flat take-up probability.
     """
+    # Skip if student_loan_plan column doesn't exist yet (e.g., during
+    # initial dataset creation before imputation runs)
+    if "student_loan_plan" not in current_year.person.columns:
+        return current_year
+
     year = int(current_year.time_period)
 
     person = current_year.person.copy()