Fix compilation on esp32

Author: tfry-git

EmbAJAX.cpp

@@ -31,6 +31,7 @@
 // statics
 EmbAJAXOutputDriverBase *EmbAJAXBase::_driver;
 char EmbAJAXBase::itoa_buf[ITOA_BUFLEN];
+constexpr const char EmbAJAXBase::null_string[1];
 
 ////////////////////////////// EmbAJAXOutputDriverBase ////////////////////
 

EmbAJAX.h

@@ -95,7 +95,7 @@ template<size_t NUM> friend class EmbAJAXContainer;
 
     static EmbAJAXOutputDriverBase *_driver;
     static char itoa_buf[8];
-    constexpr static char null_string[1] = "";
+    constexpr static const char null_string[1] = "";
 
     /** Filthy trick to keep (template) implementation out of the header. See EmbAJAXContainer::printChildren() */
     void printChildren(EmbAJAXBase** children, size_t num) const;