RFR: 8282311: Fix a typo in javax.lang.model.type.NullType

[Pavel Rappo]

Please review this trivial fix.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/7646/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7646&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282311
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7646.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7646/head:pull/7646

PR: https://git.openjdk.java.net/jdk/pull/7646