Thank you, @juanshishido! 
Ok, I found the UI page for that funding instrument, and indeed, there was no spend for that period of time, but I found another interval for which there is spend, and the API returns null:
// for placement 'PUBLISHER_NETWORK'
array(
'data_type' => 'stats',
'time_series_length' => 1,
'data' =>
array (
0 =>
array(
'id' => 'ivs1r',
'id_data' =>
array (
0 =>
array(
'segment' => NULL,
'metrics' =>
array(
'billed_charge_local_micro' => NULL,
'billed_engagements' => NULL,
),
),
),
),
),
'request' =>
array(
'params' =>
array(
'start_time' => '2019-01-31T22:00:00Z',
'segmentation_type' => NULL,
'entity_ids' =>
array (
0 => 'ivs1r',
),
'end_time' => '2019-02-06T22:00:00Z',
'country' => NULL,
'placement' => 'PUBLISHER_NETWORK',
'granularity' => 'TOTAL',
'entity' => 'FUNDING_INSTRUMENT',
'platform' => NULL,
'metric_groups' =>
array (
0 => 'BILLING',
),
),
),
)
// for placement 'ALL_ON_TWITTER'
array(
'data_type' => 'stats',
'time_series_length' => 1,
'data' =>
array (
0 =>
array(
'id' => 'ivs1r',
'id_data' =>
array (
0 =>
array(
'segment' => NULL,
'metrics' =>
array(
'billed_charge_local_micro' => NULL,
'billed_engagements' => NULL,
),
),
),
),
),
'request' =>
array(
'params' =>
array(
'start_time' => '2019-01-31T22:00:00Z',
'segmentation_type' => NULL,
'entity_ids' =>
array (
0 => 'ivs1r',
),
'end_time' => '2019-02-06T22:00:00Z',
'country' => NULL,
'placement' => 'ALL_ON_TWITTER',
'granularity' => 'TOTAL',
'entity' => 'FUNDING_INSTRUMENT',
'platform' => NULL,
'metric_groups' =>
array (
0 => 'BILLING',
),
),
),
)
As you can see, the API requests for both PUBLISHER_NETWORK and ALL_ON_TWITTER placements return null, but in the UI there is a spend of €0.02 for that period.
Thank you.