SPDX file with invalid documentNamespace value is flagged as valid by tools-java

[vargenau]

SPDX file contains:

  "documentNamespace" : "https//nokia.com",

(no ":" after https)

java-tools 1.1.8 says:

This SPDX Document is valid.

It should be flagged as invalid, it is not a valid URI.

tools-python say it is invalid.

[goneall]

@vargenau I added checks for the document namespace.

However, the example used above is actually a valid URI (an invalid URL, but a valid URI) according to the Java URI parser.

It will, however, catch other errors such as ":missingprotocol/example"