AK: Make JsonValue::from_string("") return a null JsonValue

This unbreaks the /var/run/utmp system which starts out as an empty
string, and is then turned into an object by the first update.

This isn't necessarily the best way for this to work, but it's how
it used to work, so this just fixes the regression for now.

AK/JsonValue.cpp

@@ -230,6 +230,8 @@ void JsonValue::clear()
 #ifndef KERNEL
 ErrorOr<JsonValue> JsonValue::from_string(StringView input)
 {
+    if (input.is_empty())
+        return JsonValue();
     return JsonParser(input).parse();
 }
 #endif

Tests/AK/TestJSON.cpp

@@ -122,3 +122,9 @@ TEST_CASE(json_u64_roundtrip)
     EXPECT_EQ_FORCE(value.is_error(), false);
     EXPECT_EQ(value.value().as_u64(), big_value);
 }
+
+TEST_CASE(json_parse_empty_string)
+{
+    auto value = JsonValue::from_string("");
+    EXPECT_EQ(value.value().is_null(), true);
+}