fix(process-discovery): simplify processTagsSerialized to use null directly

Address review feedback: use null instead of empty string for
processTagsSerialized when disabled, removing redundant || null
coercion. Fix test title to match assertion.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: tlhunter

packages/dd-trace/src/tracer_metadata.js

@@ -15,8 +15,8 @@ function storeConfig (config) {
     const processTags = require('./process-tags')
 
     const processTagsSerialized = config.propagateProcessTags?.enabled
-      ? (processTags.serialized || '')
-      : ''
+      ? (processTags.serialized || null)
+      : null
 
     const metadata = new processDiscovery.TracerMetadata(
       config.tags['runtime-id'],
@@ -25,7 +25,7 @@ function storeConfig (config) {
       config.service || null,
       config.env || null,
       config.version || null,
-      processTagsSerialized || null,
+      processTagsSerialized,
       containerId || null
     )
 

packages/dd-trace/test/tracer_metadata.spec.js

@@ -64,7 +64,7 @@ describe('tracer_metadata', () => {
     sinon.assert.calledOnce(storeMetadataStub)
   })
 
-  it('passes empty string for process_tags when propagateProcessTags is disabled', () => {
+  it('passes null for process_tags when propagateProcessTags is disabled', () => {
     storeConfig({ ...baseConfig, propagateProcessTags: { enabled: false } })
 
     const args = TracerMetadataStub.firstCall.args