Tests for strings
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a character vector/string.
is_nonempty_string: Test that a character is both a string (character vector of length one)
and that it is non-empty, has at least one character and is not missing.
is_optional_string: Check for an optional string: must be a character, not missing,
a vector of either length 0 or 1, and if provided must not be empty ("").
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