oepydevshell-internal.py: decode only when readdata is valid
fix below problem:
pydevshell raises exception when maximize the python shell window.
when click maximize, rlist of select return ready object, but the
pty.read is None, so throw exception of 'NoneType' object has no
attribute 'decode', change to only decode when readdata is valid.
[YOCTO #11875]
Signed-off-by: Changqing Li <changqing.li@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
