Fix fmt function to handle null pointers and return empty string

Author: mutouyun

src/libipc/imp/fmt.cpp

@@ -221,8 +221,11 @@ bool to_string(fmt_context &ctx, std::string const &a) noexcept {
 }
 
 bool to_string(fmt_context &ctx, char const *a, span<char const> fstr) noexcept {
-  if (a == nullptr) return false;
-  return ipc::sprintf(ctx, fmt_of, fstr, "s", a);
+  if (a == nullptr) {
+    return ipc::sprintf(ctx, fmt_of, fstr, "s", "");
+  } else {
+    return ipc::sprintf(ctx, fmt_of, fstr, "s", a);
+  }
 }
 
 bool to_string(fmt_context &ctx, char a) noexcept {

test/imp/test_imp_fmt.cpp

@@ -112,11 +112,10 @@ TEST(fmt, fmt) {
   EXPECT_EQ(s, test);
 
   EXPECT_EQ(ipc::fmt("", 1, "", '2', "", 3.0), "123.000000");
+  char const * nc = nullptr;
+  EXPECT_EQ(ipc::fmt(nc, 1, "", '2', "", 3.0), "123.000000");
   std::string empty;
   EXPECT_EQ(ipc::fmt(empty, 1, "", '2', "", 3.0), "123.000000");
-  EXPECT_EQ(ipc::fmt(empty, 1, empty, '2', "", 3.0), "123.000000");
-  EXPECT_EQ(ipc::fmt("", 1, empty, '2', empty, 3.0), "123.000000");
-  EXPECT_EQ(ipc::fmt("", 1, "", '2', empty), "12");
 }
 
 namespace {