RFR: 6245410: javax.swing.text.html.CSS.Attribute: BACKGROUND_POSITION is not w3c spec compliant

[Prasanta Sadhukhan]

[CSS spec for background-position](https://www.w3.org/TR/CSS22/colors.html#propdef-background-position)
mention the initial value should be "0% 0%" but JDK has the value as "null".
Rectified the spec violation. 
No regression is seen in client tests..

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Commit messages:
 - fix
 - 6245410: javax.swing.text.html.CSS.Attribute: BACKGROUND_POSITION is not w3c spec compliant

Changes: https://git.openjdk.org/jdk/pull/13114/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13114&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6245410
  Stats: 43 lines in 2 files changed: 42 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13114.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13114/head:pull/13114

PR: https://git.openjdk.org/jdk/pull/13114