Use the built in first_valid_index and last_valid_index they are designed specifically for this and slice your df:
In [5]:
first_idx = df.first_valid_index()
last_idx = df.last_valid_index()
print(first_idx, last_idx)
df.loc[first_idx:last_idx]
1950 1954
Out[5]:
sum
1950 5
1951 3
1952 NaN
1953 4
1954 8