[pdata] Fix copying of optional fields when the source is unset (#13268)

If destination has an optional field set to some value but source has it
unset, we need to unset the field on the destination instead of keeping
the old value.

Found this bug while working on
#13267

Author: dmitryax

.chloggen/pdata-fix-copy-optional-field.yaml

@@ -0,0 +1,20 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: pdata
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fix copying of optional fields when the source is unset.
+
+# One or more tracking issues or pull requests related to the change
+issues: [13268]
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]

pdata/internal/cmd/pdatagen/internal/base_fields.go

@@ -281,6 +281,10 @@ const accessorsOptionalPrimitiveTestTemplate = `func Test{{ .structName }}_{{ .f
 	{{- end }}
 	ms.Remove{{ .fieldName }}()
 	assert.False(t, ms.Has{{ .fieldName }}())
+	dest := New{{ .structName }}()
+	dest.Set{{ .fieldName }}({{ .testValue }})
+	ms.CopyTo(dest)
+	assert.False(t, dest.Has{{ .fieldName }}())
 }`
 
 type baseField interface {
@@ -860,7 +864,9 @@ func (opv *optionalPrimitiveValue) GenerateSetWithTestValue(ms *messageValueStru
 }
 
 func (opv *optionalPrimitiveValue) GenerateCopyOrig(ms *messageValueStruct) string {
-	return "if src." + opv.fieldName + "_ != nil {\n" +
+	return "if src." + opv.fieldName + "_ == nil {\n" +
+		"\tdest." + opv.fieldName + "_ = nil\n" +
+		"} else {\n" +
 		"\tdest." + opv.fieldName + "_ = &" + ms.originFullName + "_" + opv.fieldName + "{" + opv.fieldName + ": src.Get" + opv.fieldName + "()}\n" +
 		"}"
 }