Avoid creating an empty ZIP file when exporting to QIF

It may happen when there aren't transactions to export.

Author: rivaldi8

app/src/main/java/org/gnucash/android/export/qif/QifExporter.java

@@ -238,7 +238,10 @@ public List<String> generateExport() throws ExporterException {
             /// export successful
             PreferencesHelper.setLastExportTime(TimestampHelper.getTimestampFromNow());
             List<String> exportedFiles = splitQIF(file);
-            return zipQifs(exportedFiles);
+            if (exportedFiles.isEmpty())
+                return Collections.emptyList();
+            else
+                return zipQifs(exportedFiles);
         } catch (IOException e) {
             throw new ExporterException(mExportParams, e);
         }