fix: include parent in manifest when retrieving - W-18650723

[shetzel]

What does this PR do?

When retrieving a CustomObject that has an empty object-meta.xml file, include the CustomObject in the manifest entry and not just custom fields or other children. This does not affect deploying or converting since different behavior is required for those operation. I.e., including a CustomObject in the manifest without the object-meta data would fail deployment validation. See #1375

What issues does this PR fix or reference?

#3300, @W-18650723@

Functionality Before

sf project retrieve start -m CustomObject:TestObject__c with an empty object-meta.xml file would result in a manifest like this:

<Package xmlns="http://soap.sforce.com/2006/04/metadata">
    <types>
        <members>TestObject__c.TestField__c</members>
        <name>CustomField</name>
    </types>
    <version>63.0</version>
</Package>

Functionality After

<Package xmlns="http://soap.sforce.com/2006/04/metadata">
    <types>
        <members>TestObject__c.TestField__c</members>
        <name>CustomField</name>
    </types>
    <types>
        <members>TestObject__c</members>
        <name>CustomObject</name>
    </types>
    <version>63.0</version>
</Package>

[soridalac]

QA notes:
✅ : Run sf project retrieve start -m CustomObject:TestObject__c with an empty object-meta.xml file. Result with correct content of the object-meta.xml file.