[FIX] base_import: trying to import an empty file

Various bits would lead to odd tracebacks or a complete lack of
actionable feedback.

Discover that a binary field set to b'' will return None when read back,
handle that, then handle reading an empty CSV file, then handle the
resulting iterator having no lines whatsoever when trying to match
headers.

At least for CSVs, we're now properly telling users that their file
seems to have no content when they literally upload an empty file.

addons/base_import/models/base_import.py

@@ -220,7 +220,7 @@ class Import(models.TransientModel):
         """
         self.ensure_one()
         # guess mimetype from file content
-        mimetype = guess_mimetype(self.file)
+        mimetype = guess_mimetype(self.file or b'')
         (file_extension, handler, req) = FILE_TYPE_DICT.get(mimetype, (None, None, None))
         if handler:
             try:
@@ -254,7 +254,7 @@ class Import(models.TransientModel):
     @api.multi
     def _read_xls(self, options):
         """ Read file content, using xlrd lib """
-        book = xlrd.open_workbook(file_contents=self.file)
+        book = xlrd.open_workbook(file_contents=self.file or b'')
         return self._read_xls_book(book)
 
     def _read_xls_book(self, book):
@@ -298,7 +298,7 @@ class Import(models.TransientModel):
     @api.multi
     def _read_ods(self, options):
         """ Read file content using ODSReader custom lib """
-        doc = odf_ods_reader.ODSReader(file=io.BytesIO(self.file))
+        doc = odf_ods_reader.ODSReader(file=io.BytesIO(self.file or b''))
 
         return (
             row
@@ -311,7 +311,10 @@ class Import(models.TransientModel):
         """ Returns a CSV-parsed iterator of all non-empty lines in the file
             :throws csv.Error: if an error is detected during CSV parsing
         """
-        csv_data = self.file
+        csv_data = self.file or b''
+        if not csv_data:
+            return iter([])
+
         encoding = options.get('encoding')
         if not encoding:
             encoding = options['encoding'] = chardet.detect(csv_data)['encoding'].lower()
@@ -514,7 +517,10 @@ class Import(models.TransientModel):
         if not options.get('headers'):
             return [], {}
 
-        headers = next(rows)
+        headers = next(rows, None)
+        if not headers:
+            return [], {}
+
         matches = {}
         mapping_records = self.env['base_import.mapping'].search_read([('res_model', '=', self.res_model)], ['column_name', 'field_name'])
         mapping_fields = {rec['column_name']: rec['field_name'] for rec in mapping_records}