Check externalGraphic void string.

Check externalGraphic void string. There is a similar check in LabelRenderer.java
It makes possible to print clusters in vector layers like:

    var layer = new OpenLayers.Layer.Vector('MyVectorLayer', {
        styleMap: new OpenLayers.StyleMap({
            "default": new OpenLayers.Style({
                pointRadius: 10,
		externalGraphic: "${externalGraphic}",
                graphicHeight: "${graphicHeight}",
                graphicWidth: "${graphicWidth}",
	        label: "${count}",
                labelAlign: "cm",
                labelYOffset: "${labelYOffset}",
                fontSize: "13px",
                fontFamily: "arial,helvetica,clean,sans-serif",
                fontWeight: "bold",
                fillColor: "#fff",
                strokeColor: "#333",
                strokeWidth: 2,
                cursor: "pointer",
            }, {
                context: {
                    externalGraphic: function(feature) {
                        if (feature.cluster.length == 1) {
                            var type = feature.cluster[0].attributes.type;
                            switch(type) {
                            case 1: 
                                return OpenLayers.ImgPath + "img/img1.png";
                            case 2: 
                                return OpenLayers.ImgPath + "img/img2.png";
                            }
                        }
                        return '';
                    },
                    count: function(feature) {
                        return feature.attributes.count > 1 ? feature.attributes.count : '';
                    },
                    graphicWidth: function(feature) {
                        if (feature.cluster.length == 1) {
                            var type = feature.cluster[0].attributes.type;
                            if (type == 1 || type == 2 || type == 4) {
                                return 21;
                            } else if (type == 3) {
                                return 15;
                            }
                        }
                    },
                    graphicHeight: function(feature) {
                        if (feature.cluster.length == 1) {
                            var type = feature.cluster[0].attributes.type;
                            if (type == 1 || type == 2 || type == 3) {
                                return 17;
                            } else if (type == 4) {
                                return 22;
                            }
                        }
                    }
                }
            }),
        }),
        strategies: [new OpenLayers.Strategy.Fixed(), cluster],
        protocol: new OpenLayers.Protocol.Script({
            url: options.wmsURL,
            format: new OpenLayers.Format.GeoJSON(),
            params: {
                service: "WFS",
                version: "1.1.0",
                request: "GetFeature",
                typename: 'feature:' + options.layerName,
                outputformat: 'geojsonp',
                lang: OpenLayers.Lang.getCode()
            }
        })
    });