[FIX] account_extra_reports: None unattended

For the partner ledger report, since LEFT JOIN are used in the request
we could get NULL values which broke the following code.

note: issue reported on 7c6c188

addons/account_extra_reports/report/account_partner_ledger.py

@@ -28,10 +28,9 @@ class ReportPartnerLedger(models.AbstractModel):
         sum = 0.0
         for r in res:
             r['displayed_name'] = '-'.join(
-                    (r['move_name'] not in ['', '/'] and [r['move_name']] or []) +
-                    (r['ref'] not in ['', '/'] and [r['ref']] or []) +
-                    (r['name'] not in ['', '/'] and [r['name']] or [])
-                )
+                r[field_name] for field_name in ('move_name', 'ref', 'name')
+                if r[field_name] not in (None, '', '/')
+            )
             sum += r['debit'] - r['credit']
             r['progress'] = sum
             full_account.append(r)