RFR: 8291625: DialogPane without header nor headerText nor graphic node adds padding to the left of the content pane

[Jose Pereda]

This PR fixes an issue when there is a DialogPane that has no header and no graphic is set, by adding the `graphic-container` styleclass only when there is a non-null graphic applied, preventing the padding that otherwise the graphic container will keep.

Two tests have been added, to verify that the padding is 0 when there is no graphic (this test fails without this PR) and to verify that the padding is correct when there is a graphic to the left of the content area.

As part of this PR or as possible follow-up, it could be also discussed that the right/bottom padding of the graphic container shouldn't be 0 when there is no header and the graphic is laid out to the left of the content area.

-------------

Commit messages:
 - Add graphic-container styleclass only when there is a graphic

Changes: https://git.openjdk.org/jfx/pull/859/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=859&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291625
  Stats: 47 lines in 2 files changed: 40 ins; 3 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/859.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/859/head:pull/859

PR: https://git.openjdk.org/jfx/pull/859