Fixed the screen manager not being able to clear images

[NicoleYarroch]

Fixes #942

This PR is ready for review.

Risk

This PR makes no API changes.

Testing Plan

Test cases added to the ScreenManagerTests and TextAndGraphicManagerTests classes.

Summary

The blank artwork object was never created due to a bug where the blank artwork could only be created if it was already not null.

Changelog

Bug Fixes

• Developers can now successfully clear a primary or secondary graphic when using the screen manager.

CLA

• I have signed the CLA

[NicoleYarroch]

@joeygrover PR is ready for review.

[codecov-io]

Codecov Report

Merging #954 into develop will increase coverage by 0.03%.
The diff coverage is 100%.

@@              Coverage Diff              @@
##             develop     #954      +/-   ##
=============================================
+ Coverage      42.55%   42.59%   +0.03%     
- Complexity      3735     3738       +3     
=============================================
  Files            434      434              
  Lines          23098    23098              
  Branches        2698     2698              
=============================================
+ Hits            9829     9838       +9     
+ Misses         12623    12617       -6     
+ Partials         646      643       -3

Impacted Files	Coverage Δ	Complexity Δ
...icelink/managers/screen/TextAndGraphicManager.java	51.05% <100%> (+1.17%)	74 <2> (+1)	⬆️
...smartdevicelink/managers/screen/ScreenManager.java	67.36% <0%> (+4.21%)	34% <0%> (+2%)	⬆️

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