BUG: ZeroDivisionError when returning zero-length series/arrays

Cachey produces an unhelpful div/zero error when a cached function returns a zero-length array.

Examples:
```python
from cachey import Cache
import numpy as np
import pandas as pd

cache = Cache(1e5)

@cache.memoize
def myfunc(x):
return x

try:
myfunc(pd.Series())
except ZeroDivisionError as m:
print(m)

try:
myfunc(np.array([]))
except ZeroDivisionError as m:
print(m)

myfunc([]) #lists work fine
```
```console

float division by zero
float division by zero

Out[39]:
[]
```

Start by reproducing the zero-length pandas Series and NumPy array examples through Cache.memoize, then trace the cache sizing or timing path invoked by those calls. Confirm that zero-length Series and arrays no longer raise ZeroDivisionError while the existing list example continues to work.

Written by the indexing model from the issue text.