For anyone else who's having this problem and doesn't want to create a formula for every field that could possible have a blank value, the answer is indeed in the Export Format Options. I opened the file and scrolled to <Excel> then <ExcelAreaType>. Mine was originally set to 0 which means Invalid (why should this be the default?). I changed it to 255 for whole report and this seemed to fix the problem. My Excel exported reports now match the Crystal version perfectly. Also, I discovered that changing the setting <ShowGridLines> to True caused the Excel reports to now have gridlines. This has been something that has annoyed me for ages, so I'm very pleased to now have user friendly Excel reports! I have pasted the Excel portion of the code that I changed below.
P.S. I'm using Crystal Reports 2013 and Logicity Professional 1.8.8
<Excel>
<ConvertDateValuesToString>False</ConvertDateValuesToString>
<ExcelAreaGroupNumber>0</ExcelAreaGroupNumber>
<ExcelAreaType>255</ExcelAreaType> <!-- 0=Invalid, 1=ReportHeader, 2=PageHeader, 3=GroupHeader, 4=Detail, 5=GroupFooter, 7=PageFooter, 8=ReportFooter, 255=WholeReport-->
<ExcelConstantColumnWidth>36</ExcelConstantColumnWidth>
<ExcelTabHasColumnHeadings>False</ExcelTabHasColumnHeadings>
<ExcelUseConstantColumnWidth>False</ExcelUseConstantColumnWidth>
<ExportPageBreaksForEachPage>False</ExportPageBreaksForEachPage>
<ExportPageHeadersAndFooters>1</ExportPageHeadersAndFooters> <!-- 0=None, 1=OncePerReport, 2=OnEachPage-->
<FirstPageNumber>1</FirstPageNumber>
<LastPageNumber>999</LastPageNumber>
<ShowGridLines>True</ShowGridLines>
<UsePageRange>False</UsePageRange>
</Excel>