Equality checks with nothing

[devmotion]

This is only a cosmetic change (as far as I can see) since we dispatch on the types of the checked variables and hence all checks should be compiled away, but apparently using === nothing instead of == nothing can be slightly more performant (see JuliaLang/julia#29674 (comment)) and it doesn't hurt to use === consistently. Additionally, === always returns a Bool whereas e.g. missing == nothing evaluates to missing.

[coveralls]

Coverage increased (+6.9%) to 53.208% when pulling 1de500a on nothing_equality into 90c47b6 on master.

[codecov]

Codecov Report

Merging #587 into master will not change coverage.
The diff coverage is 23.07%.

@@           Coverage Diff           @@
##           master     #587   +/-   ##
=======================================
  Coverage   67.87%   67.87%           
=======================================
  Files          82       82           
  Lines       24069    24069           
=======================================
  Hits        16338    16338           
  Misses       7731     7731

Impacted Files	Coverage Δ
src/integrators/integrator_utils.jl	71.09% <0%> (ø)	⬆️
src/integrators/integrator_interface.jl	74.78% <0%> (ø)	⬆️
src/dense/generic_dense.jl	70.37% <0%> (ø)	⬆️
src/derivative_utils.jl	69.12% <100%> (ø)	⬆️
src/solve.jl	73.73% <44.44%> (ø)	⬆️

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