test/cls_lua: don't append uninitialized memory to bufferlist.

test/cls_lua: don't append uninitialized memory to bufferlist.

This is intended to fix the following compiler's warnings:

```
[1115/1786] Building CXX object src/test/cls_lua/CMakeFiles/ceph_test_cls_lua.dir/test_cls_lua.cc.o
../src/test/cls_lua/test_cls_lua.cc: In member function ‘virtual void ClsLua_Stat_Test::TestBody()’:
../src/test/cls_lua/test_cls_lua.cc:685:12: warning: ‘buf’ may be used uninitialized [-Wmaybe-uninitialized]
  685 |   bl.append(buf, sizeof(buf));
      |   ~~~~~~~~~^~~~~~~~~~~~~~~~~~
In file included from ../src/include/encoding.h:38,
                 from ../src/include/uuid.h:9,
                 from ../src/include/types.h:21,
                 from ../src/test/cls_lua/test_cls_lua.cc:3:
../src/include/buffer.h:1124:10: note: by argument 2 of type ‘const char*’ to ‘void ceph::buffer::v15_2_0::list::append(const char*, unsigned int)’ declared here
 1124 |     void append(const char *data, unsigned len);
      |          ^~~~~~
../src/test/cls_lua/test_cls_lua.cc:683:8: note: ‘buf’ declared here
  683 |   char buf[1024];
      |        ^~~
../src/test/cls_lua/test_cls_lua.cc: In member function ‘virtual void ClsLua_WriteFull_Test::TestBody()’:
../src/test/cls_lua/test_cls_lua.cc:893:14: warning: ‘buf’ may be used uninitialized [-Wmaybe-uninitialized]
  893 |   blin.append(buf, sizeof(buf));
      |   ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
In file included from ../src/include/encoding.h:38,
                 from ../src/include/uuid.h:9,
                 from ../src/include/types.h:21,
                 from ../src/test/cls_lua/test_cls_lua.cc:3:
../src/include/buffer.h:1124:10: note: by argument 2 of type ‘const char*’ to ‘void ceph::buffer::v15_2_0::list::append(const char*, unsigned int)’ declared here
 1124 |     void append(const char *data, unsigned len);
      |          ^~~~~~
../src/test/cls_lua/test_cls_lua.cc:891:8: note: ‘buf’ declared here
  891 |   char buf[1024];
```

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>