sed cannot insert if a file ends with empty line

I have a file uca.xml (Thunar configuration for custom actions):

<?xml encoding="UTF-8" version="1.0"?>
<actions>
<action>
    <!--some code-->
</action>
</actions>
<--blank line here-->

Mind that the file ends with a blank line

I want a bash command/script to insert a file customAction.txt containing:

<action>
    <!--custom configuration-->
</action>

so it will end looking like:

<?xml encoding="UTF-8" version="1.0"?>
<actions>
<action>
    <!--some code-->
</action>
<action>
    <!--custom configuration-->
</action>
</actions>

I tried a method given by jfgagne:

sed -n -i -e '/<\/actions>/r customAction.txt' -e 1x -e '2,${x;p}' -e '${x;p}' uca.xml

but it works only if there is at least one character (not a blank line) inserted below the tag.

My temporary workaround is a script:

echo "someDummyText" >> uca.xml
sed -n -i -e '/<\/actions>/r customaction.txt' -e 1x -e '2,${x;p}' -e '${x;p}' uca.xml
sed -i 's/someDummyText//' uca.xml
sed -i '${/^$/d;}' uca.xml

but I believe there is a more elegant, one-line solution with sed. Thanks in advance.

Note: this is my very post to the U&L and StackExchange community, so please be lenient with me and do not hesitate to correct me verbosely if I did something wrong. I read the SE FAQ and tried to search for an answer elsewhere with not much luck.