Fixes #469 - Prevented rst crash with only-space strings

[claudep]

Thanks nexone for the report.

[codecov[bot]]

Codecov Report

Merging #470 into master will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #470   +/-   ##
=======================================
  Coverage   90.66%   90.66%           
=======================================
  Files          28       28           
  Lines        2613     2614    +1     
=======================================
+ Hits         2369     2370    +1     
  Misses        244      244           

Impacted Files	Coverage Δ
src/tablib/formats/_rst.py	95.08% <100.00%> (ø)
tests/test_tablib.py	98.55% <100.00%> (+<0.01%)	:arrow_up:

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[hugovk]

Thanks!