Remove check for zero size in block compress (python#27)

There are valid use cases for passing a zero length string
to compress:

python-lz4/python-lz4#27

Author: jonathanunderwood

lz4/block/_block.c

@@ -121,11 +121,6 @@ compress (PyObject * Py_UNUSED (self), PyObject * args, PyObject * kwargs)
       return NULL;
     }
 
-  if (source_size <= 0) {
-    PyErr_Format(PyExc_ValueError, "Input source data size invalid: %d bytes", source_size);
-    return NULL;
-  }
-
   if (!strncmp (mode, "default", sizeof ("default")))
     {
       comp = DEFAULT;