Guard against empty strings given to select_next_proto #846

Job	Run time
rustfmt	7s
Cross build from macOS to Linux (FIPS) (x86_64-unknown-linux-gnu)	5m 36s
clippy	8m 17s
Cross build from macOS to Linux (x86_64-unknown-linux-gnu)	2m 44s
Test features	6m 58s
Test FIPS integration	3m 5s
Test (stable)	2m 10s
Test (arm-android)	2m 22s
Test (arm64-android)	2m 1s
Test (i686-android)	1m 55s
Test (x86_64-android)	2m 1s
Test (aarch64-ios)	2m 12s
Test (aarch64-ios-sim)	2m 17s
Test (x86_64-ios)	2m 7s
Test (i686-linux)	2m 38s
Test (arm-linux)	2m 29s
Test (aarch64-linux)	2m 26s
Test (arm64-macos)	2m 28s
Test (x86_64-macos)	2m 9s
Test (x86_64-mingw)	6m 46s
Test (i686-msvc)	7m 49s
Test (x86_64-msvc)	6m 9s
	1h 16m 46s

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