`add_dependencies_to_description()` adds an empty field which breaks pkg check

[benmarwick]

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Describe the bug
When we run add_dependencies_to_description() we often get an empty first field in the Imports field of the DESCRIPTION.

Imports:  ,  # <- this comma is the problem
    ape (>= 5.3),
    assertthat (>= 0.2.1),
    backports (>= 1.1.5),
    callr (>= 3.4.2),