doc: fix build with python 3.8

doc: fix build with python 3.8

After upgrading to python-3.8.0, a syntax mismatch is revealed:

doc/guides/conf.py:240: SyntaxWarning: "is not" with a literal.
    Did you mean "!="?
    if value is not '':

Removing "is not ''" seems the right thing to do.

A patch may also be needed in the RTD theme package:
https://github.com/readthedocs/sphinx_rtd_theme/commit/a49a812c.diff
(not included in release 0.4.3)

Fixes: 9db3f52126fb ("doc: generate NIC overview table from ini files")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>