ScriptInfo: use null version for missing scripts

If a script file does not actually exist, we cannot hash its contents
nor obtain a last modified timestamp. So we return null for the version.

This avoids a nasty exception dump on the log in ScriptInfoTest.

Author: ctrueden

src/main/java/org/scijava/script/ScriptInfo.java

@@ -295,6 +295,7 @@ public String getLocation() {
 	@Override
 	public String getVersion() {
 		final File file = new File(path);
+		if (!file.exists()) return null; // no version for non-existent script
 		final Date lastModified = FileUtils.getModifiedTime(file);
 		final String datestamp =
 			new SimpleDateFormat("yyyy-MM-dd-HH:mm:ss").format(lastModified);