improvement: select without placeholder

[gidomanders]

The placeholder you define cannot be disabled. If you don't specify a
placeholder, you still have an empty value you can select even if you
don't want to. It would help if Select would not display an empty option
if the placeholder is empty.

Added check if placeholder is empty to prevent an empty option.

Closes #616

[codecov]

Codecov Report

Merging #617 (30c5fb6) into master (6fe3261) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #617   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          131       131           
  Lines         2521      2523    +2     
  Branches       639       642    +3     
=========================================
+ Hits          2521      2523    +2     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/form/Select/Select.tsx	100.00% <100.00%> (ø)

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