Handle idiosyncrasy of .UsedRange on empty sheet

[bskinn]

Instead of returning something relatively intuitive, like Nothing,
when .UsedRange is called on an empty Worksheet, Excel 2019 instead
returns Range("$A$1").

Thus, since the code tries to reduce the ExportRange to the bounds
of .UsedRange when entire rows and/or columns are selected, if
Selection encompasses $A$1 then it will erroneously set ExportRange
to Range("$A$1"), instead of Nothing.

This commit fixes this behavior by running an explicit check for
.UsedRange == Range("$A$1") and IsEmpty(.UsedRange). If the
sheet is empty, then ExportRange will be set to the appropriate
Nothing.