fix: content in manifest entry should be optional

[nullccxsy]

kContent in manifest entry should be optional, not required

[wgtmac]

Make sense. Thanks!

[zhjwpku]

+1

[zeroshade]

Should there be a corresponding test for this?

[wgtmac]

@zeroshade More test cases will be added progressively because we only have a basic reader implementation:

• We are adding test manifest list and manifest files to verify manifest reader implementations are basically correct: test:  enhance ManifestListReaderV2Test and unified test architecture #168 and test: add test of manifestlistv1 #171. But this heavily depends on the generated files and not easy to produce various files.
• We are on the way to implement manifest writers so we can add round trip test to verify v1/v2/v3 compatibility. This can help test all the required/optional fields like this PR.
• We will add spark docker integration test once we can run e2e table scan.