Expand empty directories in GN to "."

tools/gn/substitution_writer_unittest.cc

Index: tools/gn/substitution_writer_unittest.cc
diff --git a/tools/gn/substitution_writer_unittest.cc b/tools/gn/substitution_writer_unittest.cc
index f17e31db996d03b82fdf968fc0f7247ad0288108..fc6cc74f9a9fc4974d5eac702cff9be458fb4abe 100644
--- a/tools/gn/substitution_writer_unittest.cc
+++ b/tools/gn/substitution_writer_unittest.cc
@@ -190,7 +190,7 @@ TEST(SubstitutionWriter, TargetSubstitutions) {
 
   EXPECT_TRUE(SubstitutionWriter::GetTargetSubstitution(
       &target, SUBSTITUTION_ROOT_OUT_DIR, &result));
-  EXPECT_EQ("", result);
+  EXPECT_EQ(".", result);
 
   EXPECT_TRUE(SubstitutionWriter::GetTargetSubstitution(
       &target, SUBSTITUTION_TARGET_GEN_DIR, &result));
@@ -253,5 +253,5 @@ TEST(SubstitutionWriter, LinkerSubstitutions) {
 
   OutputFile output = SubstitutionWriter::ApplyPatternToLinkerAsOutputFile(
       &target, tool, pattern);
-  EXPECT_EQ("libbaz.so", output.value());
+  EXPECT_EQ("./libbaz.so", output.value());
 }