Gallery Block: show InspectorControls (1 image)

[Soean]

Description

The gallery settings should also be visible if the gallery has just 1 image. There is already a check for 0 images (line 161), so we can completely remove the length-check.

How Has This Been Tested?

1. Add a Gallery Block
2. Open the block settings
3. The gallery settings should be visible.

[aduth]

Is it clear why this condition exists in the first place? Perhaps that it's odd to see the Columns control when only one image exists? Should we consider hiding that specific control when only one image exists in the set?

[codecov]

Codecov Report

Merging #3270 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3270      +/-   ##
==========================================
+ Coverage   31.02%   31.05%   +0.03%     
==========================================
  Files         232      232              
  Lines        6518     6521       +3     
  Branches     1163     1163              
==========================================
+ Hits         2022     2025       +3     
  Misses       3770     3770              
  Partials      726      726

Impacted Files	Coverage Δ
blocks/library/gallery/block.js	10% <ø> (ø)	⬆️
editor/header/preview-button/index.js	95.83% <0%> (+0.59%)	⬆️

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[Soean]

@aduth You are right, the condition was introduced in #1135 for the column slider:
c2732e7?diff=unified#diff-a7fc58c9ade4d7f0d4de7abd9f6275aaR150

A slider with min 1 and max 1 doesn't make sense. So I made an additional commit to add the conditional check just for the slider.