No VIVO_HOME/rdf directory

[chenejac]

Describe the bug
The bug is introduced by this PR. It will be probably irrelevant after merging this PR, but we should fix this meanwhile. If VIVO_HOME is empty,

                                     <delete includeemptydirs="true">
                                            <fileset dir="${vivo-dir}/rdf">
                                                <include name="**/*"/>
                                                <exclude name="**/i18n/*_x_*/**"/>
                                            </fileset>
                                        </delete>

is throwing exception (no rdf directory)

To Reproduce

1. configure setting.xml (for instance VIVO/installer/example-settings.xml) to point to an empty VIVO_HOME directory
   <vivo-dir>/opt/VIVO/home1111</vivo-dir>
2. mvn clean install -s installer/example-settings.xml
3. See error

Expected behavior
It should also work when VIVO_HOME is empty (installation for the first time).