[SchemaDumper] Handle edge cases for metric tables

(AKA:  The "Martin H. Factor" commit)

So, when doing something outside of Rails to, say, backup a metrics
table before testing a migration by doing something like:

> cop[y] metric_rollups_01 to xmetric_rollups_01 as a backup

Then this will get picked up by the `METRIC_ROLLUP_TABLE_REGEXP`, and
work though the other methods (`determine_table_parent`) and cause the
following error:

    ** Execute db:schema:dump
    rake aborted!
    NoMethodError: undefined method `[]' for nil:NilClass
    manageiq-schema/lib/manageiq/schema/schema_dumper.rb:100:in `determine_table_parent'
    manageiq-schema/lib/manageiq/schema/schema_dumper.rb:91:in `track_miq_metric_table_inheritance'
    manageiq-schema/lib/manageiq/schema/schema_dumper.rb:16:in `table'

This does another check to handle these cases.  This means said table
can't be used with the Rails models, but that should be fine since this
table is definitely outside of the Rails managed tables.

lib/manageiq/schema/schema_dumper.rb

@@ -87,8 +87,8 @@ def triggers(stream)
 
       def track_miq_metric_table_inheritance(table)
         return unless table.match?(METRIC_ROLLUP_TABLE_REGEXP)
+        return unless (inherit_from = determine_table_parent(table))
 
-        inherit_from              = determine_table_parent(table)
         inherited_metrics_tables << [table, inherit_from]
       end
 
@@ -97,12 +97,16 @@ def inherited_metrics_tables
       end
 
       def determine_table_parent(table_name)
-        @connection.execute(<<-SQL).first["parent_table"]
+        table = @connection.execute(<<-SQL)
           SELECT pg_class.relname AS parent_table
           FROM   pg_catalog.pg_inherits
           JOIN pg_class ON pg_class.oid = pg_inherits.inhparent
           WHERE inhrelid = '#{table_name}'::regclass
         SQL
+
+        return if table.first.nil?
+
+        table.first["parent_table"]
       end
 
       def column_spec_for_primary_key(column)