Fix resource sets not handling manifests correctly

[arunkumar9t2]

Proposed Changes

Previously resource set was not handling cases where res/assets folders were empty and only manifest existed for a variant. In that case the manifest was completely skipped in the generated code, this fixes it by changing the logic to handle the empty assets/res directories.

Example

    resource_sets = {
        "debug": {
            "manifest": "src/debug/AndroidManifest.xml",
        },
        "main": {
            "manifest": "src/main/AndroidManifest.xml",
        },
    },

Testing

Updated tests