NullAway 0.13.7

This is a minor bug fix release addressing a couple of issues observed in 0.13.6.

• Add support for filtering null objects from streams in #1597
• Report init warnings for JPA constructors with arguments in #1604
• Maintenance
  • Update release instructions in #1593
  • Switch code coverage aggregation to use built-in jacoco plugin by @subhramit in #1598
  • Bump Error Prone to 2.50.0 in #1601