Small FlatMap optimization with default initialization (#2620) #1579

Job	Run time
clang-compile-with-cmake	2m 57s
clang-compile-with-make	3m 4s
gcc-compile-with-bazel	18m 7s
gcc-compile-with-bazel-all-options	17m 50s
gcc-compile-with-boringssl	18m 45s
gcc-compile-with-cmake	3m 16s
gcc-compile-with-cmake-all-options	5m 19s
gcc-compile-with-make	3m 1s
gcc-compile-with-make-all-options	5m 3s
clang-compile-with-bazel	16m 19s
clang-compile-with-bazel-all-options	16m 34s
clang-compile-with-boringssl	16m 41s
clang-compile-with-cmake-all-options	5m 11s
clang-compile-with-make-all-options	5m 29s
clang-unittest	16m 8s
	2h 33m 44s

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