Test failures with Julia nightly

[gerlero]

JuliaLang/julia#49470 changed the exception thrown by expressions like Union{}[1] from a MethodError to an ArgumentError, causing the tests at

StaticArrays.jl/test/initializers.jl

Lines 61 to 64 in dce8c1c

	@test_throws MethodError Union{}[1]
	@test_throws MethodError Union{}[1 2]
	@test_throws MethodError Union{}[1; 2]
	@test_throws MethodError Union{}[1 2; 3 4]

to fail on Julia nightly (e.g. https://github.com/JuliaArrays/StaticArrays.jl/actions/runs/4845936898).