API: Refactor: Duplicate/free string arguments coming from msgpack

When receiving strings *from* msgpack, we don't need to duplicate/free since
the data only lives in the msgpack parse buffer until the end of the call.

But in order to reuse `msgpack_rpc_free_object` when sending event data(which is
sent *to* msgpack), Strings must be freed, which means they must also be
allocated separately.

src/nvim/api/buffer.c

@@ -307,7 +307,7 @@ Integer buffer_get_number(Buffer buffer, Error *err)
 
 String buffer_get_name(Buffer buffer, Error *err)
 {
-  String rv = {.size = 0, .data = ""};
+  String rv = STRING_INIT;
   buf_T *buf = find_buffer(buffer, err);
 
   if (!buf || buf->b_ffname == NULL) {

src/nvim/api/private/defs.h

@@ -6,6 +6,7 @@
 #include <string.h>
 
 #define ARRAY_DICT_INIT {.size = 0, .items = NULL}
+#define STRING_INIT {.data = NULL, .size = 0}
 #define REMOTE_TYPE(type) typedef uint64_t type
 
 #define TYPED_ARRAY_OF(type)                                                  \

src/nvim/api/private/helpers.c

@@ -319,7 +319,7 @@ tabpage_T * find_tab(Tabpage tabpage, Error *err)
 
 String cstr_to_string(const char *str) {
     if (str == NULL) {
-        return (String) { .data = NULL, .size = 0 };
+        return (String) STRING_INIT;
     }
 
     size_t len = strlen(str);