Speed up empty String creation slightly (#6573)

As @DirkMuller found out in #6568, there is a difference in code
executed between `String str(nullptr)` and `String str("")`, but in the
end the actual object is identical.  It's a few bytes of code, but every
little bit counts.

Update the default `String()` constructor to use `nullptr` and not `""`.
This will remove a constant literal load and the execution of the
String::copy method and strlen().

cores/esp8266/WString.h

@@ -53,7 +53,7 @@ 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(const char *cstr = nullptr);
         String(const String &str);
         String(const __FlashStringHelper *str);
 #ifdef __GXX_EXPERIMENTAL_CXX0X__