|Or, assuming A1 is really empty, you could try:|
=IF(A1<>"", A1+45, "")
If A1 might contain something other than a date, such that A1+45 might return an error, you could try:
=IF(ISERROR(A1+45), "", A1+45)
It might even take a combination of all three suggestions to make sure you never get an error or "obscure date".
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