Fix require nothing bug

[lassepe]

I recently hit an edge case in LazySets.jl where a @require Package="<UUID>" nothing statement would not hit the add_require dispatch in Revise because the nothing is parsed as a Symbol, not an Expression. I have thus widened the type constraint in add_expression to accept Union{Expr,Symbol} and adjusted the body to always convert to an Expr first.

The test that I added fails on master but works with this PR. However, there may be a more meaningful test to write here. I'm not too familiar with the test structure here.

[lassepe]

I have no idea why the CI fails on this. The tests for the changes that I made pass locally for me.

[timholy]

Presumably you need to add it as an [extras] and [targets] dependency?

Revise.jl/Project.toml

Lines 28 to 44 in ca3b5cf

	[extras]
	CatIndices = "aafaddc9-749c-510e-ac4f-586e18779b91"
	EndpointRanges = "340492b5-2a47-5f55-813d-aca7ddf97656"
	EponymTuples = "97e2ac4a-e175-5f49-beb1-4d6866a6cdc3"
	Example = "7876af07-990d-54b4-ab0e-23690620f79a"
	IndirectArrays = "9b13fd28-a010-5f03-acff-a1bbcff69959"
	InteractiveUtils = "b77e0a4c-d291-57a0-90e8-8db25a27a240"
	MacroTools = "1914dd2f-81c6-5fcd-8719-6d5c9610ff09"
	MappedArrays = "dbb5928d-eab1-5f90-85c2-b9b0edb7c900"
	Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"
	Requires = "ae029012-a4dd-5104-9daa-d747884805df"
	RoundingIntegers = "d5f540fe-1c90-5db3-b776-2e2f362d9394"
	Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
	UnsafeArrays = "c4a57d5a-5b31-53a6-b365-19f8c011fbd6"
	[targets]
	test = ["CatIndices", "EndpointRanges", "EponymTuples", "Example", "IndirectArrays", "InteractiveUtils", "MacroTools", "MappedArrays", "Random", "Requires", "RoundingIntegers", "Test", "UnsafeArrays"]

But one issue is that your new test passes for me even on Revise master, so I don't think it's testing what you think it is.