Merge pull request #7280 from Goober5000/campaign_optional_string_fred #921

Job	Run time
Linux (Release, gcc-13)	15m 10s
Linux (Debug, gcc-9)	17m 4s
Linux (Debug, gcc-13)	11m 0s
Linux (Release, gcc-13, -DFSO_BUILD_WITH_OPENGL=OFF -DFSO_BUILD_WITH_VULKAN=OFF)	16m 26s
Linux (Debug, clang-16)	14m 31s
Linux (Debug, gcc-13, -DFSO_BUILD_WITH_OPENGL=OFF -DFSO_BUILD_WITH_VULKAN=OFF)	14m 34s
Linux (Release, gcc-9)	21m 0s
Linux (Release, clang-16)	21m 37s
	2h 11m 22s

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