fix: support NULL values for key labels

[burningalchemist]

Description

At the moment, if at least one of the columns specified in key_labels contains NULL in a row, the query will return the error and the metrics won't be published (for a single target). It's by design of database/sql - NULL is not an empty string, so it cannot be converted to a string out of the box.

We actually want to support NULL values for metric labels (strings). If a row contains NULL in a column that's defined in key_labels, we return its value as an empty string. In addition we create a log (under debug log level) message, that the column value is NULL for a particular row.

---

Example

Sample Dataset:

Username	Identifier	Firstname	Lastname
	9012	Rachel	Booker
grey07	2070	Laura	Grey
johnson81	4081	Craig	Johnson
jenkins46	9346	Mary	Jenkins
smith79	5079	Jamie	Smith

Metric configuration:

- metric_name: test
  type: gauge
  values: [Identifier]
  key_labels:
  - Username
  - Firstname
  - Lastname
  query: |
    SET @@DELIMITER TO ';';
    SELECT Username, Firstname, Lastname, Identifier FROM `username.csv`

The result:

# HELP test
# TYPE test gauge
test{Firstname="Craig",Lastname="Johnson",Username="johnson81",instance="test"} 4081
test{Firstname="Jamie",Lastname="Smith",Username="smith79",instance="test"} 5079
test{Firstname="Laura",Lastname="Grey",Username="grey07",instance="test"} 2070
test{Firstname="Mary",Lastname="Jenkins",Username="jenkins46",instance="test"} 9346
test{Firstname="Rachel",Lastname="Booker",Username="",instance="test"} 9012

fixes #317