String: Provide sensible default constructor

- Provide a sensible constructor to create a blank String, without copying a zero-length string.
- Also provide a simple constructor that accepts a single char.

Author: MattiasTF

cores/esp32/WString.cpp

@@ -30,6 +30,11 @@
 /*  Constructors                             */
 /*********************************************/
 
+String::String() {
+  init();
+  reserve(0); // This will create a valid zero-length string with SSO enabled.
+}
+
 String::String(const char *cstr) {
   init();
   if (cstr) {
@@ -63,8 +68,7 @@ String::String(StringSumHelper &&rval) {
 
 String::String(char c) {
   init();
-  char buf[] = {c, '\0'};
-  *this = buf;
+  copy(&c, 1);
 }
 
 String::String(unsigned char value, unsigned char base) {

cores/esp32/WString.h

@@ -54,7 +54,8 @@ class String {
   // if the initial value is null or invalid, or if memory allocation
   // fails, the string will be marked as invalid (i.e. "if (s)" will
   // be false).
-  String(const char *cstr = "");
+  String();
+  String(const char *cstr);
   String(const char *cstr, unsigned int length);
 #ifdef __GXX_EXPERIMENTAL_CXX0X__
   String(const uint8_t *cstr, unsigned int length) : String(reinterpret_cast<const char *>(cstr), length) {}