BUG: read_excel fails when empty sheets exist and sheetname=None #11711
Fixes issue #11711.

Existing code prematurely returns an empty dataframe when an empty sheet in the source excel file is encountered.
Fix is to store an empty dataframe in the output dict and continue to the next sheet.

does this cover both cases in #11711 (IIRC blank sheet in middle was ok, at end was buggy)

but was there another case?
 This should cover cases where blank sheets are anywhere in the excel file, including in the middle, as stated in the #11711 [comment](https://github.com/pydata/pandas/issues/11711#issuecomment-160151977). 
 ok lgtm. ping when green.
 build done & green
 thank you!
