Upgrade NullAway 0.10.19 -> 0.10.21 (#975)

See:
- https://github.com/uber/NullAway/blob/master/CHANGELOG.md
- https://github.com/uber/NullAway/releases/tag/v0.10.20
- https://github.com/uber/NullAway/releases/tag/v0.10.21
- uber/NullAway@v0.10.19...v0.10.21

pom.xml

@@ -212,7 +212,7 @@
         <version.maven>3.8.7</version.maven>
         <version.mockito>5.8.0</version.mockito>
         <version.nopen-checker>1.0.1</version.nopen-checker>
-        <version.nullaway>0.10.19</version.nullaway>
+        <version.nullaway>0.10.21</version.nullaway>
         <version.pitest-git>1.1.4</version.pitest-git>
         <version.rewrite-templating>1.4.2</version.rewrite-templating>
         <version.surefire>3.2.3</version.surefire>