RFR: 8047749: javadoc for getPathBounds() in TreeUI and BasicTreeUI is incorrect

[Prasanta Sadhukhan]

The javadoc descriptions for javax.swing.plaf.TreeUI.getPathBounds() and javax.swing.plaf.basic.BasicTreeUI.getPathBounds() have stated it returns null if any component in path is currently "valid", which is wrong.
The getPathBounds() in BasicTreeUI returns null if tree or treeState is null. 
Fixed the javadoc.

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Commit messages:
 - 8047749: javadoc for getPathBounds() in TreeUI and BasicTreeUI is incorrect

Changes: https://git.openjdk.java.net/jdk/pull/7421/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7421&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8047749
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7421.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7421/head:pull/7421

PR: https://git.openjdk.java.net/jdk/pull/7421