RFR: 8265909 : build.tools.dtdbuilder.DTDBuilder.java failed detecting missing path of dtd_home

[ScientificWare]

This concern [dtdbuilder tools](https://github.com/openjdk/jdk/tree/master/make/jdk/src/classes/build/tools/dtdbuilder).

In jshell, try `System.getProperty("html32") + ""` you'll get a `String`.

So, in `DTDBuilder.java` when none `dtd_home` property is set, the `dtd_home` string value is not `null`, causing an exception with the present test.

The expect value is `"Must set property 'dtd_home'"`

And in this case, should'nt we have a `System.exit(1)` too rather than a simple `return` ?

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Commit messages:
 - Change test none dtd_home property is set.

Changes: https://git.openjdk.java.net/jdk/pull/3626/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3626&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265909
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3626.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3626/head:pull/3626

PR: https://git.openjdk.java.net/jdk/pull/3626