ETL-772: Fix/dlq empty response

[PabloPardoGarcia]

• Fix json decoding error when consuming events from dlq and there are no events
• Fix model validation error when getting pipeline config with null transformation

[github-actions]

Test Coverage Report

File	Stmts	Miss	Cover	Missing
etl
__init__.py	5	0	100%
api_client.py	64	5	92%	70–73, 123
client.py	39	4	89%	59–61, 141
dlq.py	41	1	97%	70
errors.py	27	0	100%
pipeline.py	146	7	95%	334–337, 350, 406–407
tracking.py	26	0	100%
utils.py	11	11	0%	1, 3, 6, 8–11, 17–18, 22, 28
etl/models
__init__.py	11	0	100%
base.py	10	4	60%	7–10
config.py	34	8	76%	37, 40–41, 43–45, 47–48
data_types.py	80	0	100%
filter.py	17	2	88%	16, 22
join.py	51	2	96%	44, 80
metadata.py	4	0	100%
pipeline.py	98	2	97%	156, 159
schema.py	26	0	100%
sink.py	61	7	88%	33, 39, 47, 49, 53, 55, 61
source.py	132	7	94%	47, 51, 68, 81, 124, 150, 166
stateless_transformation.py	46	8	82%	44, 48, 52, 57, 69, 71, 73, 80
TOTAL	929	68	92%