* contrib/hbwin/olecore.c
+ added functions which changes default method of NIL conversion to
OLE variant:
__oleVariantNil2Null( [<lNewNil2NullFlag>] ) -> <lPrevNil2NullFlag>
By default NIL values passed to foreign OLE code are converted to
VT_EMPTY variants. __oleVariantNil2Null( .t. ) changes this behavior
so NIL values are converted to VT_NULL variants.
; Warning: This function was added for tests. Looks that it's expected
behavior for communication with MS-Excel and allows to use
NIL as default parameter value but potentially it may create
problems with some custom OLE interfaces so this modification
should be widely tested in real life before we change the
default behavior.
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