postgresql: fix last sum() related parse errors
authorMarc Fournier <marc.fournier@camptocamp.com>
Wed, 8 Nov 2017 16:26:51 +0000 (17:26 +0100)
committerMarc Fournier <marc.fournier@camptocamp.com>
Wed, 8 Nov 2017 20:32:35 +0000 (21:32 +0100)
These queries return empty values when a database doesn't have any
tables yet, which results in collectd logging this error every Interval:

```
db query utils: udb_result_submit: Parsing `' as derive failed.
```

Related to #1905


No differences found