fix bug when checking empty file. add more coverage

[llazzaro]

While adding some coverage discovered the using len(pickle) will raise and error since for empty files pickle is None.

Change the if condition to just not pickle since it will also work for checking if the list is empty.

Added two test cases to cover more branches.

[codecov[bot]]

Codecov Report

Merging #535 (4a17e3f) into master (0a3511f) will increase coverage by 0.18%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #535      +/-   ##
==========================================
+ Coverage   90.97%   91.16%   +0.18%     
==========================================
  Files          28       28              
  Lines        2660     2670      +10     
==========================================
+ Hits         2420     2434      +14     
+ Misses        240      236       -4     

Impacted Files	Coverage Δ
src/tablib/core.py	85.01% <100.00%> (+0.93%)	:arrow_up:
tests/test_tablib.py	98.66% <100.00%> (+0.01%)	:arrow_up:

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more

[claudep]

Maybe just adapt the commit message to specify this concerns the yaml format.

[claudep]

Ah yes, NamedTemporaryFile constructor is already returning a file-like object, so no need to reopen later (and Windows cannot reopen when it's already open).

[llazzaro]

• git commit message fixed
• fix windows broken test