Return reference instead of non-null pointer

[kimci86]

Since #2486 and #2494, Sprite and Text instances cannot hold a null pointer to the corresponding resource.
I suggest that getTexture and getFont return a reference instead of a pointer to make it clear that it is not null.
What do you think?

Note this is slightly changing Sprite constructor behavior. If I understand correctly, Sprite(texture, IntRect()) is equivalent to Sprite(texture) in master, whereas Sprite(texture, IntRect()) would actually create a Sprite with an empty texture rectangle with this PR. It looks to me this would be the correct behavior one would expect.

I also reviewed corresponding tests and fixed a small inconsistency in Text tests while at it.

The same kind of change would apply to Sound and getSoundBuffer once the default constructor gets removed as planned in #2507.

[codecov]

Codecov Report

Merging #2637 (371af2b) into master (954fc65) will increase coverage by 0.00%.
The diff coverage is 0.00%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #2637    +/-   ##
========================================
  Coverage   28.73%   28.73%            
========================================
  Files         229      229            
  Lines       19755    19753     -2     
  Branches     4725     4724     -1     
========================================
  Hits         5676     5676            
- Misses      13437    13584   +147     
+ Partials      642      493   -149     

Files Changed	Coverage Δ
include/SFML/Graphics/Sprite.hpp	0.00% <ø> (ø)
include/SFML/Graphics/Text.hpp	0.00% <ø> (ø)
src/SFML/Graphics/Sprite.cpp	0.00% <0.00%> (ø)
src/SFML/Graphics/Text.cpp	0.00% <0.00%> (ø)

... and 23 files with indirect coverage changes

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[ChrisThrasher]

I really like this! It removes yet another potential source of bugs from the API and further reinforces the idea that SFML 3 is systematically eliminating invalid states from being representable in the type system.

[ChrisThrasher]

https://github.com/SFML/imgui-sfml/pull/250/files

Already found a real world example this is helpful. It's useful to know that a sprite always has a texture. It saves you some manual error checking.