fix(lookup): handle null target values in XLS lookup table export

Write empty string instead of calling .as(String.class) on a null Value
when exporting rows with no mapped target value.

Fixes #959

Author: adamnadolny-wizipisi

io/plugins/eu.esdihumboldt.hale.io.xls/src/eu/esdihumboldt/hale/io/xls/writer/XLSLookupTableWriter.java

@@ -133,7 +133,7 @@ else if (getContentType().getId().equals("eu.esdihumboldt.hale.io.xls.xlsx")) {
 
 			Value entry = table.get(key);
 			cell = row.createCell(1);
-			cell.setCellValue(entry.as(String.class));
+			cell.setCellValue((entry != null) ? entry.as(String.class) : "");
 			cell.setCellStyle(rowStyle);
 			rownum++;
 		}