Many Lisp implementations on line-buffered systems do not discard the
newline that the user must type after the last right parenthesis in order
for the line to be transmitted from the OS to Lisp.  Lisp's READ function
returns immediately after seeing the matching ")" in the stream.  When
READLINE is called, it sees the next character in the stream, which is a
newline, so it returns an empty line.  If you were to type "(read-line)This
is a test" the result would be "This is a test".

The simplest solution is to use (PROGN (CLEAR-INPUT) (READ-LINE)).  This
discards the buffered newline before reading the input.  However, it would
also discard any other buffered input, as in the "This is a test" example
above; some implementation also flush the OS's input buffers, so typeahead
might be thrown away.