Fix bugs in handling of delta encodings

[etseidl]

Description

Part of #14938 was fixing two bugs discovered during testing. One is in the encoding of DELTA_BINARY_PACKED data where the first non-null value in a page to be encoded is not in the first batch of 129 values. The second is an error in decoding of DELTA_BYTE_ARRAY pages where, again, the first non-null value is not in the first block to be decoded.

This PR includes a test for the former, but the latter cannot be easily tested because the python API still lacks skip_rows, and we cannot generate DELTA_BYTE_ARRAY encoded data without the changes in #14938. A test for the latter will be added later, but the fix has been validated with data on hand locally.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.

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[vuule]

/ok to test

[davidwendt]

/ok to test

[vuule]

/merge