Crash when doing "is (not) null" checks on JSON fields

[e-gineer]

Describe the bug

There is a crash when doing null checks against JSON fields:

> select name, tags from aws_s3_bucket where tags is null
+---------------------------+--------+
| name                      | tags   |
+---------------------------+--------+
| turbot-example-us-east-2  | <null> |
+---------------------------+--------+
> 
> select name, tags from aws_s3_bucket where tags->>'Collective' = 'brooklyn'
+----------------------------+------------------------------------------------------------------------------+
| name                       | tags                                                                         |
+----------------------------+------------------------------------------------------------------------------+
| brooklyn-example-us-east-2 | {"Collective":"brooklyn","Drone":"primary","Name":"turbot.brooklyn.primary"} |
+----------------------------+------------------------------------------------------------------------------+
> 
> select name, tags from aws_s3_bucket where tags->>'Collective' is not null
Error: runtime error: invalid memory address or nil pointer dereference
> 
> select name, tags from aws_s3_bucket where tags->>'Collective' is null
Error: runtime error: invalid memory address or nil pointer dereference
> 
> 

Steampipe version (steampipe -v)

v0.3.0

To reproduce

See above

Expected behavior

Should match against the query (not crash).

Additional context

From the log:

2021-03-19 15:31:44.729 EDT [6791] LOG:  statement: select * from aws_s3_bucket
2021-03-19 15:31:48.243 EDT [6791] LOG:  duration: 3514.316 ms
2021-03-19 15:32:05.649 EDT [6791] LOG:  statement: select name, tags from aws_s3_bucket
2021-03-19 15:32:17.806 EDT [6791] LOG:  statement: select name, tags from aws_s3_bucket where tags->'Collective' is null
2021-03-19T15:32:17.806-0400 [WARN]  hub: goFdwBeginForeignScan failed with panic: runtime error: invalid memory address or nil pointer dereference
2021-03-19 15:32:17.806 EDT [6791] ERROR:  runtime error: invalid memory address or nil pointer dereference