Improve CQ7

[iamcarbon]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

• Improves a few error messages
• Replaces == null with is null (== is overloadable, and can be used on Spans)
• Throws NRE directly in trival contexts (avoiding the call overhead + expanded callstack on exception)
• Annotates a few additional fields as readonly

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #678 into master will decrease coverage by 0.01%.
The diff coverage is 53.96%.

@@            Coverage Diff             @@
##           master     #678      +/-   ##
==========================================
- Coverage   85.18%   85.17%   -0.02%     
==========================================
  Files         878      878              
  Lines       39122    39090      -32     
  Branches     2539     2539              
==========================================
- Hits        33328    33293      -35     
- Misses       5101     5104       +3     
  Partials      693      693

Impacted Files	Coverage Δ
...rc/ImageSharp/MetaData/Profiles/Exif/ExifWriter.cs	84.82% <ø> (ø)	⬆️
src/ImageSharp/Formats/Bmp/BmpDecoder.cs	100% <ø> (ø)	⬆️
.../Processing/Transforms/ProjectiveTransformTests.cs	97.22% <0%> (ø)	⬆️
...C/TagDataEntries/IccTextDescriptionTagDataEntry.cs	51.61% <0%> (ø)	⬆️
...rc/ImageSharp/PixelFormats/ColorBuilder{TPixel}.cs	84% <0%> (ø)	⬆️
...rofiles/ICC/DataWriter/IccDataWriter.Primitives.cs	89.79% <0%> (ø)	⬆️
...files/ICC/TagDataEntries/IccCrdInfoTagDataEntry.cs	66.66% <0%> (ø)	⬆️
...mageSharp/MetaData/Profiles/ICC/IccTagDataEntry.cs	53.33% <0%> (ø)	⬆️
...aData/Profiles/Exif/ExifTagDescriptionAttribute.cs	53.84% <0%> (ø)	⬆️
...rofiles/ICC/TagDataEntries/IccCurveTagDataEntry.cs	56% <0%> (ø)	⬆️
... and 68 more

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[iamcarbon]

Ready for review.

[JimBobSquarePants]

We should keep an eye out for more things like this but I suspect you already have it covered 👍