ogrinfo: add -skip-null

[jidanni]

Feature description

I'm not sure the following feature request is worthwhile, because the user could always just grep -v the results that are bothering them, but here goes anyway:

Sometimes half a file might be filled with nulls,

$ ogrinfo $@ -al | grep --fixed-strings --count ' = '
512910
$ ogrinfo $@ -al | grep --fixed-strings --count ' = (null)'
263320

You know, page upon page of

FRSTDIVNO (String) = 22
FRSTDIVDUP (String) = 0
FRSTDIVLAB (String) = 22
SECDIVID (String) = IL020180N0140W0SN220
SECDIVTYP (String) = (null)
SECDIVTXT (String) = (null)
SECDIVNO (String) = (null)
SECDIVSUF (String) = (null)
SECDIVNOTE (String) = (null)
SECDIVLAB (String) = (null)
SURVID (String) = (null)
SURVTYP (String) = (null)
SURVTYPTXT (String) = (null)

So maybe have a -nonull or -skip-null option, that would act like

| grep --fixed-strings --invert-match ' = (null)'

i.e., leaving just the

FRSTDIVNO (String) = 22
FRSTDIVDUP (String) = 0
FRSTDIVLAB (String) = 22
SECDIVID (String) = IL020180N0140W0SN220

without needing an additional grep process, or some massive -sql or -where, with sub-quires.