fix clang++ warning

src/sip/CreateWcsWithSip.cc: In constructor 'lsst::meas::astrom::sip::CreateWcsWithSip<MatchT>::CreateWcsWithSip(const std::vector<T>&, const lsst::afw::geom::SkyWcs&, int, const lsst::geom::Box2I&, int)':
src/sip/CreateWcsWithSip.cc:145:46: warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses]
     if (_bbox.isEmpty() && !_matches.empty() > 0) {
                                              ^

Looks like a '_matches.size() > 0' was only half-changed to
'!_matches.empty()'.

src/sip/CreateWcsWithSip.cc

@@ -142,7 +142,7 @@ CreateWcsWithSip<MatchT>::CreateWcsWithSip(std::vector<MatchT> const& matches,
      * If no BBox is provided, guess one from the input points (extrapolated a bit to allow for fact
      * that a finite number of points won't reach to the edge of the image)
      */
-    if (_bbox.isEmpty() && !_matches.empty() > 0) {
+    if (_bbox.isEmpty() && !_matches.empty()) {
         for (typename std::vector<MatchT>::const_iterator ptr = _matches.begin(); ptr != _matches.end();
              ++ptr) {
             afw::table::SourceRecord const& src = *ptr->second;