fix: handle null parent spans in asChildOf calls

[eddie4941]

Motivation:

The opentracing apis state that when calling the asChildOf methods with
null parameters they should behave as no-ops. The existing code was
throwing null pointers in these cases

ref: https://github.com/opentracing/opentracing-java/blob/master/opentracing-api/src/main/java/io/opentracing/Tracer.java#L150

Modifications:

• treat null values in childOf calls as no-ops

Result:

Our tracer implementation adheres to the opentracing apis

[Scottmitch]

our call sites guard against calling this method with null. Can you update these to avoid the double null check in TracingHttpServiceFilter and TracingHttpRequesterFilter?

[Scottmitch]

there are some check style issues related to import ordering. You can also run ./gradlew checkstyle locally to validate.

Error: eckstyle] [ERROR] /home/runner/work/servicetalk/servicetalk/servicetalk-opentracing-inmemory/src/test/java/io/servicetalk/opentracing/inmemory/DefaultInMemoryTracerTest.java:19:1: Import statement for 'io.servicetalk.opentracing.inmemory.api.InMemoryReference' is in the wrong order. Should be in the 'SPECIAL_IMPORTS' group, expecting group 'STANDARD_JAVA_PACKAGE' on this line. [CustomImportOrder]
[1505](https://github.com/apple/servicetalk/runs/5761785777?check_suite_focus=true#step:6:1505)

[1506](https://github.com/apple/servicetalk/runs/5761785777?check_suite_focus=true#step:6:1506)
> Task :servicetalk-opentracing-inmemory:checkstyleTest FAILED
[1507](https://github.com/apple/servicetalk/runs/5761785777?check_suite_focus=true#step:6:1507)
Error: eckstyle] [ERROR] /home/runner/work/servicetalk/servicetalk/servicetalk-opentracing-inmemory/src/test/java/io/servicetalk/opentracing/inmemory/DefaultInMemoryTracerTest.java:20:1: Import statement for 'io.servicetalk.opentracing.inmemory.api.InMemoryScopeManager' is in the wrong order. Should be in the 'SPECIAL_IMPORTS' group, expecting group 'STANDARD_JAVA_PACKAGE' on this line. [CustomImportOrder]
[1508](https://github.com/apple/servicetalk/runs/5761785777?check_suite_focus=true#step:6:1508)
Error: eckstyle] [ERROR] /home/runner/work/servicetalk/servicetalk/servicetalk-opentracing-inmemory/src/test/java/io/servicetalk/opentracing/inmemory/DefaultInMemoryTracerTest.java:21:1: Import statement for 'io.servicetalk.opentracing.inmemory.api.InMemorySpan' is in the wrong order. Should be in the 'SPECIAL_IMPORTS' group, expecting group 'STANDARD_JAVA_PACKAGE' on this line. [CustomImportOrder]
[1509](https://github.com/apple/servicetalk/runs/5761785777?check_suite_focus=true#step:6:1509)
Error: eckstyle] [ERROR] /home/runner/work/servicetalk/servicetalk/servicetalk-opentracing-inmemory/src/test/java/io/servicetalk/opentracing/inmemory/DefaultInMemoryTracerTest.java:22:1: Import statement for 'io.servicetalk.opentracing.inmemory.api.InMemorySpanBuilder' is in the wrong order. Should be in the 'SPECIAL_IMPORTS' group, expecting group 'STANDARD_JAVA_PACKAGE' on this line. [CustomImportOrder]
[1510](https://github.com/apple/servicetalk/runs/5761785777?check_suite_focus=true#step:6:1510)
Error: eckstyle] [ERROR] /home/runner/work/servicetalk/servicetalk/servicetalk-opentracing-inmemory/src/test/java/io/servicetalk/opentracing/inmemory/DefaultInMemoryTracerTest.java:23:1: Import statement for 'io.servicetalk.opentracing.inmemory.api.InMemorySpanContext' is in the wrong order. Should be in the 'SPECIAL_IMPORTS' group, expecting group 'STANDARD_JAVA_PACKAGE' on this line. [CustomImportOrder]
[1511](https://github.com/apple/servicetalk/runs/5761785777?check_suite_focus=true#step:6:1511)
Error: eckstyle] [ERROR] /home/runner/work/servicetalk/servicetalk/servicetalk-opentracing-inmemory/src/test/java/io/servicetalk/opentracing/inmemory/DefaultInMemoryTracerTest.java:24:1: Import statement for 'io.servicetalk.opentracing.inmemory.api.InMemoryTracer' is in the wrong order. Should be in the 'SPECIAL_IMPORTS' group, expecting group 'STANDARD_JAVA_PACKAGE' on this line. [CustomImportOrder]

[Scottmitch]

thanks @eddie4941 !