Fix NullPointerException in cquery --output=graph.

[gregestren]

Fixes #12915.

The repro from the bug crashes because these two deps are compared:

Target: @remote_java_tools_linux//:java_tools/src/tools/singlejar/singlejar_local
Config: d8d0eb07eb92791668ac973282be1523379b0025b22f0ade56b13d519f2feb2a

Target: @remote_java_tools_linux//:java_tools/src/tools/singlejar/singlejar_local
Config: null

This is a cc_binary:

bazel/src/tools/singlejar/BUILD

Lines 92 to 93 in 0c1257e

	cc_binary(
	name = "singlejar_local",

I have no idea why the graph visitor tags one of its instances with a null configuration. Nevertheless, this code is still safer.

[katre]

This is the same as the compareTo below, and the checksum2 null check isn't needed, either. I'd simplify this as:

if (checksum1 == null) {
return -1;
} else {
return checksum1.compareTo(checksum2);
}

[gregestren]

How's this? I left the double null check because of this comment from https://docs.oracle.com/javase/8/docs/api/java/lang/Comparable.html:

Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false.

[gregestren]

Or am I missing something about the null check?

[katre]

Ah, I hadn't realized that. Okay, the second null check is necessary, then.

[gregestren]

Also note I don't know how to add a test since I don't know how to invoke a dep simultaneously in a null and non-null configuration.

It's probably worth exploring how the query logic decides a cc_binary appears in both configurations. Not sure I should worry about that for the scope of this PR (scope creep). But open to suggestions.