Reproducing Step:

1. open zss.test/issue3/457-emptyHyperlink.zul

Actual:
Exception shows up:

java.lang.NullPointerException
at org.zkoss.zss.ui.impl.CellFormatHelper.escapeText(CellFormatHelper.java:531)
at org.zkoss.zss.ui.impl.CellFormatHelper.getHyperlinkHtml(CellFormatHelper.java:441)
at org.zkoss.zss.ui.impl.CellFormatHelper.getRichCellHtmlText(CellFormatHelper.java:431)
at org.zkoss.zssex.ui.impl.CellDisplayLoaderImpl.getCellHtmlText(CellDisplayLoaderImpl.java:35)
at org.zkoss.zss.ui.Spreadsheet$ExtraCtrl.getCellAttr(Spreadsheet.java:3332)
at org.zkoss.zss.ui.Spreadsheet$ExtraCtrl.getRangeAttrs(Spreadsheet.java:3164)
at org.zkoss.zss.ui.Spreadsheet.createActiveRange(Spreadsheet.java:1728)
at org.zkoss.zss.ui.Spreadsheet.renderProperties(Spreadsheet.java:1688)
at org.zkoss.zk.ui.AbstractComponent.redraw(AbstractComponent.java:1807)
at org.zkoss.zk.ui.AbstractComponent.redrawChildren(AbstractComponent.java:1884)