Fix handling of empty string_view (#689)

include/fmt/format.h

@@ -1399,8 +1399,9 @@ class arg_formatter_base {
   }
 
   void write(const char_type *value) {
-    auto length = value != FMT_NULL ?
-      std::char_traits<char_type>::length(value) : 0;
+    if (!value)
+      FMT_THROW(format_error("string pointer is null"));
+    auto length = std::char_traits<char_type>::length(value);
     writer_.write_str(basic_string_view<char_type>(value, length), specs_);
   }
 
@@ -2647,8 +2648,6 @@ void basic_writer<Range>::write_str(
   internal::char_traits<char_type>::convert(Char());
   const Char *data = s.data();
   std::size_t size = s.size();
-  if (size == 0 && !data)
-    FMT_THROW(format_error("string pointer is null"));
   std::size_t precision = static_cast<std::size_t>(spec.precision_);
   if (spec.precision_ >= 0 && precision < size)
     size = precision;

test/format-test.cc

@@ -1196,7 +1196,8 @@ TEST(FormatterTest, FormatString) {
 }
 
 TEST(FormatterTest, FormatStringView) {
-  EXPECT_EQ("test", format("{0}", string_view("test")));
+  EXPECT_EQ("test", format("{}", string_view("test")));
+  EXPECT_EQ("", format("{}", string_view()));
 }
 
 #ifdef FMT_USE_STD_STRING_VIEW