ec2 inventory: python 3.7 compatibility (ansible#43716)

ec2 inventory script was throwing errors when using Python 3.7:

TypeError: option values must be strings

This changes the None ConfigParser options to empty strings instead.

(cherry picked from commit 505ce6c)

contrib/inventory/ec2.py

@@ -187,16 +187,16 @@
     'all_elasticache_replication_groups': 'False',
     'all_instances': 'False',
     'all_rds_instances': 'False',
-    'aws_access_key_id': None,
-    'aws_secret_access_key': None,
-    'aws_security_token': None,
-    'boto_profile': None,
+    'aws_access_key_id': '',
+    'aws_secret_access_key': '',
+    'aws_security_token': '',
+    'boto_profile': '',
     'cache_max_age': '300',
     'cache_path': '~/.ansible/tmp',
     'destination_variable': 'public_dns_name',
     'elasticache': 'True',
     'eucalyptus': 'False',
-    'eucalyptus_host': None,
+    'eucalyptus_host': '',
     'expand_csv_tags': 'False',
     'group_by_ami_id': 'True',
     'group_by_availability_zone': 'True',
@@ -218,19 +218,19 @@
     'group_by_tag_keys': 'True',
     'group_by_tag_none': 'True',
     'group_by_vpc_id': 'True',
-    'hostname_variable': None,
-    'iam_role': None,
+    'hostname_variable': '',
+    'iam_role': '',
     'include_rds_clusters': 'False',
     'nested_groups': 'False',
-    'pattern_exclude': None,
-    'pattern_include': None,
+    'pattern_exclude': '',
+    'pattern_include': '',
     'rds': 'False',
     'regions': 'all',
     'regions_exclude': 'us-gov-west-1, cn-north-1',
     'replace_dash_in_groups': 'True',
     'route53': 'False',
     'route53_excluded_zones': '',
-    'route53_hostnames': None,
+    'route53_hostnames': '',
     'stack_filters': 'False',
     'vpc_destination_variable': 'ip_address'
 }