Ensure that FixedInt and FixedUInt handle zero-length integers #2825

Job	Run time
Setup Build Matrix (true, ion-rust, windows, macos, ubuntu, al2-x86, al2-arm, windows, macos, ubu...	1s
Build and Test (windows, default)	3m 8s
Build and Test (windows, all)	2m 44s
Build and Test (windows, experimental-ion-hash)	3m 11s
Build and Test (windows, experimental)	3m 19s
Build and Test (macos, default)	1m 52s
Build and Test (macos, all)	1m 17s
Build and Test (macos, experimental-ion-hash)	1m 10s
Build and Test (macos, experimental)	1m 10s
Build and Test (ubuntu, default)	1m 20s
Build and Test (ubuntu, all)	2m 5s
Build and Test (ubuntu, experimental-ion-hash)	1m 24s
Build and Test (ubuntu, experimental)	1m 18s
Build and Test (al2-x86, default)	1m 9s
Build and Test (al2-x86, all)	1m 10s
Build and Test (al2-x86, experimental-ion-hash)	1m 4s
Build and Test (al2-x86, experimental)	1m 8s
Build and Test (al2-arm, default)	1m 25s
Build and Test (al2-arm, all)	1m 26s
Build and Test (al2-arm, experimental-ion-hash)	1m 27s
Build and Test (al2-arm, experimental)	1m 23s
Build and Test Confirmation	1s
	34m 12s

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