fix(Dropdown): omit moveSelectionBy() when options aren't defined

[layershifter]

Fixes #1875.

[codecov-io]

Codecov Report

Merging #2119 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2119   +/-   ##
=======================================
  Coverage   99.76%   99.76%           
=======================================
  Files         150      150           
  Lines        2596     2596           
=======================================
  Hits         2590     2590           
  Misses          6        6

Impacted Files	Coverage Δ
src/modules/Dropdown/Dropdown.js	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c003fe9...8978737. Read the comment docs.

[levithomason]

Since this is already submitted, I'll merge. However, I want to avoid any fixes for using children with the Dropdown and instead direct users to use the options prop.

I'm afraid we'll end up on a slippery slope of fixing small issues with children constantly, thereby supporting it.

[levithomason]

Released in semantic-ui-react@0.74.2