Fixed JsonVariant::is<int>() that returned true for empty strings

CHANGELOG.md

@@ -5,6 +5,7 @@ HEAD
 ----
 
 * Improved float serialization when `-fsingle-precision-constant` is used
+* Fixed `JsonVariant::is<int>()` that returned true for empty strings
 
 v5.13.2
 -------

src/ArduinoJson/Polyfills/isInteger.hpp

@@ -10,10 +10,10 @@ namespace ArduinoJson {
 namespace Internals {
 
 inline bool isInteger(const char* s) {
-  if (!s) return false;
+  if (!s || !*s) return false;
   if (issign(*s)) s++;
   while (isdigit(*s)) s++;
   return *s == '\0';
 }
-}
-}
+}  // namespace Internals
+}  // namespace ArduinoJson

test/Polyfills/isInteger.cpp

@@ -12,6 +12,10 @@ TEST_CASE("isInteger()") {
     REQUIRE_FALSE(isInteger(NULL));
   }
 
+  SECTION("Empty String") {
+    REQUIRE_FALSE(isInteger(""));
+  }
+
   SECTION("FloatNotInteger") {
     REQUIRE_FALSE(isInteger("3.14"));
     REQUIRE_FALSE(isInteger("-3.14"));