Add a static const String::ms_empty empty string

src/string.cc

@@ -10,6 +10,8 @@
 namespace Kakoune
 {
 
+const String String::ms_empty;
+
 Vector<String> split(StringView str, char separator, char escape)
 {
     Vector<String> res;

src/string.hh

@@ -124,6 +124,8 @@ public:
     void resize(ByteCount size) { m_data.resize((size_t)(int)size); }
     void reserve(ByteCount size) { m_data.reserve((size_t)(int)size); }
 
+    static const String ms_empty;
+
 private:
     Content m_data;
 };