fix Series.count (#336)

* fix Series.count

* fix Series.count

Author: Rubtsowa

sdc/datatypes/hpat_pandas_series_functions.py

@@ -2955,7 +2955,8 @@ def hpat_pandas_series_count(self, level=None):
     if isinstance(self.data, StringArrayType):
         def hpat_pandas_series_count_str_impl(self, level=None):
 
-            return len(self._data)
+            nan_mask = self.isna()
+            return numpy.sum(nan_mask._data == 0)
 
         return hpat_pandas_series_count_str_impl