Fix sql-insert format emits NULL as 'None' (#1409)

* Sub: Fix issue #1408

Body:
1. Fix issue #1408 sql-insert format emits NULL as 'None';
2. Fix DUAL displays as ""DUAL"";

==== End ====

* Sub: Update changelog.rst

Body:

==== End ====

* Sub: Optimize if logic

Body:

==== End ====

2 files changed, 9 insertions, 4 deletions

diff --git a/pgcli/auth.py b/pgcli/auth.py
index 3d2fbeb9..2f1e5526 100644
--- a/pgcli/auth.py
+++ b/pgcli/auth.py
@@ -26,7 +26,9 @@ def keyring_initialize(keyring_enabled, *, logger):
 
         try:
             keyring = importlib.import_module("keyring")
-        except ModuleNotFoundError as e:
+        except (
+            ModuleNotFoundError
+        ) as e:  # ImportError for Python 2, ModuleNotFoundError for Python 3
             logger.warning("import keyring failed: %r.", e)
 
 
diff --git a/pgcli/packages/formatter/sqlformatter.py b/pgcli/packages/formatter/sqlformatter.py
index 5bf25fec..5224eff6 100644
--- a/pgcli/packages/formatter/sqlformatter.py
+++ b/pgcli/packages/formatter/sqlformatter.py
@@ -14,10 +14,13 @@ preprocessors = ()
 
 
 def escape_for_sql_statement(value):
+    if value is None:
+        return "NULL"
+
     if isinstance(value, bytes):
         return f"X'{value.hex()}'"
-    else:
-        return "'{}'".format(value)
+
+    return "'{}'".format(value)
 
 
 def adapter(data, headers, table_format=None, **kwargs):
@@ -29,7 +32,7 @@ def adapter(data, headers, table_format=None, **kwargs):
         else:
             table_name = table[1]
     else:
-        table_name = '"DUAL"'
+        table_name = "DUAL"
     if table_format == "sql-insert":
         h = '", "'.join(headers)
         yield 'INSERT INTO "{}" ("{}") VALUES'.format(table_name, h)