R1:67498f33340a

Restore phutil_split_lines() to Differential

Summary:
Same as D3759 with a fix:

Previously, we would insert `null` to indicate a line that doesn't exist on one side of a diff, and then implode on "\n", so we'd get "\n" as a result.

In D3759, we'd insert `null` but implode on empty string, and get nothing as a result.

When a placeholder null is present, explicitly insert a newline.

Test Plan: D3759 plus examined a diff with removed lines on the left side prior to new lines on the right side.

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