only list map records that include tile data in ZXYStream()

.gitignore

@@ -1,2 +1,4 @@
 node_modules
 test/fixtures/non_existent.mbtiles
+test/fixtures/empty.mbtiles
+test/fixtures/output

lib/zxystream.js

@@ -31,7 +31,10 @@ ZXYStream.prototype._read = function() {
 
     // Prepare sql statement
     if (!stream.statement) {
-        stream.statement = this.source._db.prepare('SELECT zoom_level AS z, tile_column AS x, tile_row AS y FROM ' + this.table, function(err) {
+        var query = 'SELECT zoom_level AS z, tile_column AS x, tile_row AS y FROM ' + this.table;
+        if (this.table === 'map') query += ' WHERE tile_id is not null';
+
+        stream.statement = this.source._db.prepare(query, function(err) {
             if (err && err.code === 'SQLITE_ERROR' && /no such table/.test(err.message)) return stream.push(null);
             return stream._read();
         });

test/zxystream.js

@@ -145,3 +145,20 @@ tape('zxystream empty zxystream', function(assert) {
         assert.end();
     });
 });
+
+tape('zxystream ignores map coords without tile_ids', function(assert) {
+    new MBTiles(__dirname + '/fixtures/some-empty-tiles.mbtiles', function(err, src) {
+        if (err) throw err;
+        var stream = src.createZXYStream();
+        var called = 0;
+        stream.on('data', function(lines) {
+            lines.toString().split('\n').forEach(function(coord) {
+                if (coord) called++;
+            });
+        });
+        stream.on('end', function() {
+            assert.equal(called, 11, 'found correct number of tiles');
+            assert.end();
+        });
+    });
+});