[alxndrsn]

Closes #97

What has been done to verify that this works as intended?

No tests, just ran it locally:

./gradlew jar &&
cat src/test/resources/xml_external_instance.xml | java -jar ./build/libs/ODK-Validate-null.jar - &&
java -jar ./build/libs/ODK-Validate-null.jar &&
java -jar ./build/libs/ODK-Validate-null.jar src/test/resources/xml_external_instance.xml &&
echo -e "\n\n--- Expect this one to fail:\n" &&
echo booboo | java -jar ./build/libs/ODK-Validate-null.jar -
Starting a Gradle Daemon, 1 busy and 1 incompatible and 1 stopped Daemons could not be reused, use --status for details
> Task :generateBuildConfig UP-TO-DATE
> Task :compileBuildConfig UP-TO-DATE
> Task :compileJava UP-TO-DATE
> Task :processResources UP-TO-DATE
> Task :classes UP-TO-DATE
> Task :jar UP-TO-DATE

BUILD SUCCESSFUL in 2s
5 actionable tasks: 5 up-to-date


>> Xform parsing completed!



>> Xform is valid!
Validating: src/test/resources/xml_external_instance.xml


>> Xform parsing completed!



>> Xform is valid!


--- Expect this one to fail:

[Fatal Error] :1:1: Content is not allowed in prolog.



>> XML is invalid.
org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 1; Content is not allowed in prolog.
	at java.xml/com.sun.org.apache.xerces.internal.parsers.DOMParser.parse(DOMParser.java:262)
	at java.xml/com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderImpl.parse(DocumentBuilderImpl.java:342)
	at java.xml/javax.xml.parsers.DocumentBuilder.parse(DocumentBuilder.java:122)
	at org.opendatakit.validate.FormValidator.validate(FormValidator.java:434)
	at org.opendatakit.validate.FormValidator.validate(FormValidator.java:422)
	at org.opendatakit.validate.FormValidator.main(FormValidator.java:131)

Why is this the best possible solution? Were any other approaches considered?

Are there any risks to merging this code? If so, what are they?