Fix printing of empty info strings.

Handle printing of `info string` in a single place.

Fixes #5386

closes https://github.com/official-stockfish/Stockfish/pull/5387

No functional change

src/misc.h

@@ -101,6 +101,7 @@ inline std::vector<std::string> split(const std::string& s, const std::string& d
 }
 
 void remove_whitespace(std::string& s);
+bool is_whitespace(const std::string& s);
 
 enum SyncCout {
     IO_LOCK,
@@ -111,6 +112,9 @@ std::ostream& operator<<(std::ostream&, SyncCout);
 #define sync_cout std::cout << IO_LOCK
 #define sync_endl std::endl << IO_UNLOCK
 
+void sync_cout_start();
+void sync_cout_end();
+
 // True if and only if the binary is compiled on a little-endian machine
 static inline const union {
     uint32_t i;