Reproduced error in functional test with a small sample plugin.

Without fix, Jenkins startup fails:
java.lang.NullPointerException
	at java.io.Reader.<init>(Reader.java:78)
	at java.io.InputStreamReader.<init>(InputStreamReader.java:113)
	at org.apache.commons.io.IOUtils.copy(IOUtils.java:2440)
	at org.apache.commons.io.IOUtils.toString(IOUtils.java:1084)
	at io.jenkins.plugins.loads_resource.Main.stuff(Main.java:36)