Allow Lua SEXPs to not specify a subcategory #3162

Job	Run time
Mac (Debug, clang, x86_64)	14m 59s
Linux (Debug, gcc-5)	8m 47s
Mac (Release, clang, x86_64)	10m 41s
Windows (Debug, MSVC, Win32)	24m 3s
Linux (Debug, gcc-10)	12m 29s
Windows (Debug, MSVC, x64)	20m 22s
Linux (Debug, clang-9)	16m 1s
Windows (Release, MSVC, Win32)	25m 25s
Linux (Release, gcc-5)	7m 52s
Windows (Release, MSVC, x64)	34m 3s
Linux (Release, gcc-10)	8m 14s
Linux (Release, clang-9)	16m 40s
Linux (Debug, gcc-10, -DFSO_BUILD_WITH_OPENGL=OFF -DFSO_BUILD_WITH_VULKAN=OFF)	10m 21s
Linux (Release, gcc-10, -DFSO_BUILD_WITH_OPENGL=OFF -DFSO_BUILD_WITH_VULKAN=OFF)	8m 25s
	3h 38m 22s

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