Fix NPE in JVMFlagAccess.setValue when getStringFlag returns null

[matsumo-and]

What Does This Do

Fixes a NullPointerException in JVMFlagAccess.setValue inside crash tracking initialization.
setValue calls getStringFlag after setStringFlag to verify the write succeeded, but
getStringFlag can return null on certain platforms where the JVM flag is not accessible via
the native ddprof API. The fix replaces .equals() with Objects.equals to handle null safely.

Motivation

On Apple Silicon (aarch64) and potentially other platforms, JVM flags like OnError and
OnOutOfMemoryError are not readable via JVMAccess.Flags.getStringFlag, which returns null.
This caused a NullPointerException logged as a WARN during crash tracking initialization on
every agent startup, polluting logs and causing noise in tests that assert no NPE appears.

Additional Notes

• Only JVMFlagAccess (native path) is affected. JMXFlagAccess always returns true and is unaffected.
• Returning false when getStringFlag returns null is semantically correct: it signals that
  the flag could not be verified as set, which is the expected behavior when the native API does
  not support the flag.

Contributor Checklist

• Format the title according to the contribution guidelines
• Assign the type: and (comp: or inst:) labels in addition to any other useful labels
• Avoid using close, fix, or any linking keywords when referencing an issue
  Use solves instead, and assign the PR milestone to the issue
• Update the CODEOWNERS file on source file addition, migration, or deletion
• Update public documentation with any new configuration flags or behaviors