Guard against empty strings given to select_next_proto #847

Job	Run time
clippy	3m 11s
Cross build from macOS to Linux (x86_64-unknown-linux-gnu)	2m 21s
rustfmt	12s
Cross build from macOS to Linux (FIPS) (x86_64-unknown-linux-gnu)	4m 37s
Test features	6m 48s
Test FIPS integration	3m 13s
Test (stable)	2m 3s
Test (arm-android)	2m 11s
Test (arm64-android)	2m 0s
Test (i686-android)	1m 54s
Test (x86_64-android)	4m 53s
Test (aarch64-ios)	2m 34s
Test (aarch64-ios-sim)	2m 2s
Test (x86_64-ios)	2m 21s
Test (i686-linux)	2m 20s
Test (arm-linux)	2m 24s
Test (aarch64-linux)	2m 36s
Test (arm64-macos)	2m 3s
Test (x86_64-macos)	2m 38s
Test (x86_64-mingw)	5m 40s
Test (i686-msvc)	5m 43s
Test (x86_64-msvc)	5m 43s
	1h 9m 27s

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