sysutils/e2fsprogs: handle PORTREVISION==0 in *_DEPENDS

Else we would depend on, say, e2fsprogs-core-1.48.0_ (trailing
underscore) character when PORTREVISION was zeroed or omitted.
(Note that bsd.port.mk will default it to 0 value).

Use make(1)'s varname:?trueval:falseval: expansion to guard
the expansion. (_SUF1 is private in bsd.port.mk, let's avoid that).

Reported by: Tomoaki AOKI
PR: 276643