Add method for path === nothing

[dahong67]

This makes it possible to pass a path of nothing that disables the caching functionality (both saving and loading).

The motivating usecase is instances where we only want to cache in certain circumstances. For example, we may do a sweep on a cluster where we cache each run, then want to cache the combined results but only when we've loaded the full sweep (added this example to README in #17).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d3a9e1d) to head (4c3783c).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #16   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           32        35    +3     
=========================================
+ Hits            32        35    +3     

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