sample-webapp throws java.lang.IllegalArgumentException

[axelrose]

Just some robustness on the sample webapp would be nice

If I open a fresh http://localhost:8080/jodconverter-sample-webapp/ and push "Convert!" without select some input file I get:

java.lang.IllegalArgumentException: Prefix string "" too short: length must be at least 3
        at java.base/java.io.File.createTempFile(File.java:2065)
        at java.base/java.io.File.createTempFile(File.java:2136)
        at org.jodconverter.sample.webapp.ConverterServlet.doPost(ConverterServlet.java:81)

Perhaps the code should just check for uploadedFile == "" too.

Thanks for making this such a nice project!
Axel.

[sbraconnier]

Fixed.