Fix display of empty tables

[nschoellhorn]

Before this patch, empty tables with titles were displayed very "buggy", no titles were shown and only some lines were drawn. With this patch, the titles appear and the body looks empty.

This is how tables with titles but no body looked before:

++
||
++
++

After this patch, the same table would look like this:

+---------+---------+
| Title 1 | Title 2 |
+---------+---------+
+---------+---------+

[codecov]

Codecov Report

Merging #127 into master will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
+ Coverage   85.89%   86.03%   +0.13%     
==========================================
  Files           5        5              
  Lines        1326     1339      +13     
==========================================
+ Hits         1139     1152      +13     
  Misses        187      187              

Impacted Files	Coverage Δ
src/lib.rs	87.87% <100.00%> (+0.30%)	⬆️

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[pinkforest]

Thanks sounds reasonable for 1.0.0