fix: 增加对N的判断，避免N为负数或0导致不通过

Author: waitpersistence

solution/0100-0199/0177.Nth Highest Salary/Solution.py

@@ -2,6 +2,8 @@
 
 
 def nth_highest_salary(employee: pd.DataFrame, N: int) -> pd.DataFrame:
+    if N < 1:
+        return pd.DataFrame({"getNthHighestSalary(" + str(N) + ")": [None]})
     unique_salaries = employee.salary.unique()
     if len(unique_salaries) < N:
         return pd.DataFrame([np.NaN], columns=[f"getNthHighestSalary({N})"])