Fix for #2488 BorderStyle.None now writes correctly

[OssianEPPlus]

Fix for #2488

This bug is arguably a variant of #450 AKA (s224) as it pertains to Epplus reading Empty border elements in the dxf record
e.g:

<border>
  <left/>
  <right/>
  <top/>
  <bottom/>
</border>

Adding "None" to a dxf border-style did not write any nodes to the .xml

This caused Conditional-Formattings to be unable to "format" cell-border styles as they did not contain the correct node.

The issue at its core was that we considered the setting of the style "None" to be equivalent to "null" for the "HasValue" variable.

I applied a patch via checking if the nodes for individual borders exist via AtLeastOneBorderExists() But we should consider if "HasValue" should be true for "none" values. It appears to be a re-occuring problem in multiple places in Epplus where Excel Considers an empty Node to be a Default Value but we consider and Empty Node to be "HasValue == False"