RFR: 6753661: JFileChooser font not reset after Look & Feel change [v6]

[Prasanta Sadhukhan]

> Issue is observed that after changing the Look & Feel from Metal to Nimbus and back to Metal, the Nimbus font continues to be used by a JFileChooser.
> This is because Synth  `installDefaults `methods set the font, but its inverse methods `uninstallDefaults `do not remove them.
> Fix is made to reset the font if it is set by L&F.

Prasanta Sadhukhan has updated the pull request incrementally with one additional commit since the last revision:

  Add both null check

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12180/files
  - new: https://git.openjdk.org/jdk/pull/12180/files/dbeb36dc..d4713ff2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12180&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12180&range=04-05

  Stats: 4 lines in 1 file changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12180.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12180/head:pull/12180

PR: https://git.openjdk.org/jdk/pull/12180