fix: negative timestamp in note_formatter_util (close #38)

[favoyang]

When the timestamp is negative, set it to zero.

[codecov[bot]]

Codecov Report

Merging #48 (c8374f4) into master (e09a805) will not change coverage. Report is 1 commits behind head on master. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #48   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines         1355      1365   +10     
=========================================
+ Hits          1355      1365   +10     

Files	Coverage Δ
evernote_backup/note_formatter_util.py	100.00% <100.00%> (ø)

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[vzhd1701]

Evernote supports negative timestamps since you can manually set any creation date for your notes https://dev.evernote.com/doc/reference/Types.html#Typedef_Timestamp