Only create rules/categorized categories/graduated ranges if the
associated symbol could be converted to SLD, and is not an "empty"
symbol.
Otherwise we do not generate a rule, as SLD spec requires a
Symbolizer element to be present.
sip doesn't use the standard Python staticmethod type for defining
static methods, which means that standard means of testing
for a static method (like `isinstance(..., staticmethod)`) fail
with any PyQGIS static methods.
This causes issues with lint tools, which incorrectly flag
calls to QGIS static methods as missing self arguments. It also
breaks detection of static methods in the sphinx PyQGIS docs,
so all static methods are shown as non-static.
Work around this in sipify, by wrapping unambiguously static
methods in staticmethod wrappers.
