Adding default String() constructor to ensure init() is called.

[atanisoft]

Without a default constructor it is possible to crash the esp32 with a heap corruption since the invalidate() method called by the destructor expects init() to have been called previously.

Example backtrace decode showing the failure:

0x4008c285: invoke_abort at ...\framework-espidf\components\esp32/panic.c:687
0x4008c4f5: abort at ...\framework-espidf\components\esp32/panic.c:687
0x4008b753: multi_heap_assert at ...\framework-espidf\components\heap/multi_heap.c:699
0x4008b77d: get_prev_free_block at ...\framework-espidf\components\heap/multi_heap.c:699
0x4008bb96: multi_heap_free_impl at ...\framework-espidf\components\heap/multi_heap.c:699
0x4008bf4e: multi_heap_free at ...\framework-espidf\components\heap/multi_heap_poisoning.c:301
0x40084b81: heap_caps_free at ...\framework-espidf\components\heap/heap_caps.c:354
0x40087c79: _free_r at ...\framework-espidf\components\newlib/syscalls.c:42
0x401ab476: String::~String() at ...\framework-arduinoespressif32\cores\esp32/WString.cpp:840 (discriminator 2)
0x4011ad45: AsyncWebHeader::~AsyncWebHeader() at ...\toolchain-xtensa32\xtensa-esp32-elf\include\c++\5.2.0/functional:1974
  \-> inlined by: operator() at ...\ESPAsyncWebServer\src/WebRequest.cpp:56
  \-> inlined by: _M_invoke at ...\toolchain-xtensa32\xtensa-esp32-elf\include\c++\5.2.0/functional:1871