avoid UB in StringKeyedCommon for default constructed StringPiece

avoid UB in StringKeyedCommon for default constructed StringPiece

Summary:
ubsan flags this problem:

```
buck-out/opt-ubsan/gen/folly/__default_headers__#header-mode-symlink-tree-with-header-map,headers/folly/experimental/StringKeyedCommon.h:31:18: runtime error: null pointer passed as argument 2, which is declared to never be null
third-party-buck/gcc-4.9-glibc-2.20-fb/build/glibc/include/string.h:47:28: note: nonnull attribute specified here
    #0 0x5cb88f in std::pair<std::__detail::_Node_iterator<std::pair<folly::Range<char const*> const, facebook::eden::GitIgnore>, false, true>, bool> folly::StringKeyedUnorderedMap<facebook::eden::GitIgnore, folly::Hash, std::equal_to<folly::Range<char const*> >, std::allocator<std::pair<folly::Range<char const*> const, facebook::eden::GitIgnore> > >::emplace<facebook::eden::GitIgnore>(folly::Range<char const*>, facebook::eden::GitIgnore&&) buck-out/opt-ubsan/gen/folly/__default_headers__#header-mode-symlink-tree-with-header-map,headers/folly/experimental/StringKeyedCommon.h:31
    #1 0x5c6652 in facebook::eden::(anonymous namespace)::IgnoreChecker::isIgnored(facebook::eden::detail::RelativePathBase<folly::Range<char const*> >) eden/fs/inodes/Dirstate.cpp:226
    #2 0x5c6037 in facebook::eden::(anonymous namespace)::IgnoreChecker::isIgnored(facebook::eden::detail::RelativePathBase<folly::Range<char const*> >) eden/fs/inodes/Dirstate.cpp:180
    #3 0x5c5173 in facebook::eden::Dirstate::getStatusForExistingDirectory(facebook::eden::detail::RelativePathBase<folly::Range<char const*> >) const eden/fs/inodes/Dirstate.cpp:372
    #4 0x5c4509 in facebook::eden::Dirstate::getStatus() const eden/fs/inodes/Dirstate.cpp:272
    #5 0x50a448 in verifyExpectedDirstate(facebook::eden::Dirstate const*, std::unordered_map<std::basic_fbstring<char, std::char_traits<char>, std::allocator<char>, std::fbstring_core<char> >, facebook::eden::StatusCode, std::hash<std::basic_fbstring<char, std::char_traits<char>, std::allocator<char>, std::fbstring_core<char> > >, std::equal_to<std::basic_fbstring<char, std::char_traits<char>, std::allocator<char>, std::fbstring_core<char> > >, std::allocator<std::pair<std::basic_fbstring<char, std::char_traits<char>, std::allocator<char>, std::fbstring_core<char> > const, facebook::eden::StatusCode> > >&&) eden/fs/inodes/test/DirstateTest.cpp:48
    #6 0x50c70c in Dirstate_addDirectoriesWithMixOfFiles_Test::TestBody() eden/fs/inodes/test/DirstateTest.cpp:220
    #7 0xb45857 in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/engshare/third-party2/gtest/1.7.0/src/gtest-1.7.0/./src/gtest.cc:2364
    #8 0xb36784 in testing::Test::Run() /home/engshare/third-party2/gtest/1.7.0/src/gtest-1.7.0/./src/gtest.cc:2437
    #9 0xb36957 in testing::TestInfo::Run() [clone .part.558] /home/engshare/third-party2/gtest/1.7.0/src/gtest-1.7.0/./src/gtest.cc:2612
    #10 0xb36b74 in testing::TestCase::Run() [clone .part.559] /home/engshare/third-party2/gtest/1.7.0/src/gtest-1.7.0/./src/gtest.cc:2587
    #11 0xb3806e in testing::internal::UnitTestImpl::RunAllTests() [clone .part.561] /home/engshare/third-party2/gtest/1.7.0/src/gtest-1.7.0/./src/gtest.cc:4571
    #12 0xb382d9 in testing::UnitTest::Run() /home/engshare/third-party2/gtest/1.7.0/src/gtest-1.7.0/./src/gtest.cc:4519
    #13 0x861ca7 in main third-party-buck/gcc-4.9-glibc-2.20-fb/build/gtest/include/gtest/gtest.h:2326
    #14 0x7f20be0740f5 in __libc_start_main /home/engshare/third-party2/glibc/2.20/src/glibc-2.20/csu/libc-start.c:289
    #15 0x4a552c in _start /home/engshare/third-party2/glibc/2.20/src/glibc-2.20/csu/../sysdeps/x86_64/start.S:122

UndefinedBehaviorSanitizer: undefined-behavior buck-out/opt-ubsan/gen/folly/__default_headers__#header-mode-symlink-tree-with-header-map,headers/folly/experimental/StringKeyedCommon.h:31:18
```

The issue is that `StringPiece` default constructs to `{nullptr, nullptr}` as a valid representation of an empty string.
This is tripping up UBSAN in this case.

The fix is a trivial nullptr check

Reviewed By: igorsugak

Differential Revision: D4791015

fbshipit-source-id: dec7484b29ecb29c17b8dd6a9b0e8093f07d63cb