Excludes can be None in proxy

[ianturton]

In core/proxy.py:

26 excludes = settings.value("proxy/proxyExcludedUrls", "")
27 if hasattr(excludes, 'isNull') and excludes.isNull():
28 excludes = []
29 else:
30 excludes = excludes.split("|")

If excludes in None then line 30 throws an exception.
fix to:

26 excludes = settings.value("proxy/proxyExcludedUrls", "")
27 if not excludes or (hasattr(excludes, 'isNull') and excludes.isNull()):
28 excludes = []
29 else:
30 excludes = excludes.split("|")

Environment

QGIS version	3.6.3-Noosa	QGIS code revision	0c5774c
Compiled against Qt	5.9.5	Running against Qt	5.9.5
Compiled against GDAL/OGR	2.2.3	Running against GDAL/OGR	2.2.3
Compiled against GEOS	3.6.2-CAPI-1.10.2	Running against GEOS	3.6.2-CAPI-1.10.2 4d2925d6
PostgreSQL Client Version	10.8 (Ubuntu 10.8-0ubuntu0.18.04.1)	SpatiaLite Version	4.3.0a
QWT Version	6.1.3	QScintilla2 Version	2.10.2
PROJ.4 Version	493

[Guts]

Anterior to the duplicated #88, but less documented.
Please follow up the new one.