fix error msg for empty type

[chenyan-dfinity]

No description provided.

[github-actions]

Benchmark for fa484f7

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Test	Base	PR	%
Blob/&str	248.8±69.06µs	235.5±70.07µs	-5.35%
Blob/ByteBuf	156.0±46.58µs	165.6±42.83µs	+6.15%
Blob/Bytes	119.1±41.30µs	125.2±41.50µs	+5.12%
Blob/String	318.5±152.77µs	309.7±150.76µs	-2.76%
Collections/vec (text, nat)	71.6±1.44ms	70.9±1.16ms	-0.98%
Collections/vec int	32.1±0.14ms	32.2±0.18ms	+0.31%
Collections/vec int64	19.7±0.36ms	17.7±0.35ms	-10.15%
Collections/vec nat8	14.1±0.04ms	14.3±0.03ms	+1.42%
option list/1024	1382.8±6.94µs	1372.0±3.40µs	-0.78%
profiles/1024	2.7±0.02ms	2.7±0.03ms	0.00%
variant list/1024	1124.5±14.15µs	1119.3±2.30µs	-0.46%

[github-actions]

Benchmark for 1170441

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Test	Base	PR	%
Blob/&str	254.8±70.22µs	254.9±71.77µs	+0.04%
Blob/ByteBuf	167.6±44.28µs	169.6±44.17µs	+1.19%
Blob/Bytes	115.5±30.56µs	117.9±37.44µs	+2.08%
Blob/String	260.5±70.12µs	257.6±69.00µs	-1.11%
Collections/vec (text, nat)	72.1±1.32ms	72.1±1.25ms	0.00%
Collections/vec int	32.0±0.05ms	32.8±0.16ms	+2.50%
Collections/vec int64	19.4±0.08ms	19.8±0.14ms	+2.06%
Collections/vec nat8	13.8±0.12ms	13.9±0.07ms	+0.72%
option list/1024	1375.5±6.38µs	1384.6±2.41µs	+0.66%
profiles/1024	2.7±0.02ms	2.7±0.01ms	0.00%
variant list/1024	1121.8±3.83µs	1126.7±14.32µs	+0.44%