chore(rln): return empty metadata if it doesnt exist

[rymnc]

Previously, we returned an error if the metadata field was empty in the db. Now we return an empty vec so that clients can handle the error appropriately.
See: waku-org/nwaku#2471

[github-actions]

Benchmark for 43d5bf9

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Test	Base	PR	%
Pmtree::compute_root	0.0±0.00ns	0.0±0.00ns	NaN%
Pmtree::get	317.9±4.22ns	318.0±3.31ns	+0.03%
Pmtree::override_range	236.4±3.74µs	234.4±6.47µs	-0.85%
Pmtree::set	55.7±0.75µs	56.0±2.16µs	+0.54%
Pmtree:delete	55.6±0.63µs	55.8±1.26µs	+0.36%

[github-actions]

Benchmark for 43d5bf9

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Test	Base	PR	%
FullMerkleTree::compute_root	0.0±0.00ns	0.0±0.00ns	NaN%
FullMerkleTree::delete	896.2±10.30ns	894.8±19.48ns	-0.16%
FullMerkleTree::get	3.4±0.06ns	3.4±0.04ns	0.00%
FullMerkleTree::override_range	2.2±0.02µs	2.2±0.03µs	0.00%
FullMerkleTree::set	886.3±17.77ns	883.2±11.34ns	-0.35%
OptimalMerkleTree::compute_root	1046.2±17.87ns	1046.6±38.86ns	+0.04%
OptimalMerkleTree::delete	1077.6±63.93ns	1040.7±14.61ns	-3.42%
OptimalMerkleTree::get	23.1±0.23ns	23.2±0.28ns	+0.43%
OptimalMerkleTree::override_range	5.4±0.09µs	5.3±0.17µs	-1.85%
OptimalMerkleTree::set	1047.0±8.90ns	1050.7±64.82ns	+0.35%

[github-actions]

Benchmark for eea865e

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Test	Base	PR	%
Pmtree::compute_root	0.0±0.00ns	0.0±0.00ns	NaN%
Pmtree::get	321.7±4.45ns	320.6±3.87ns	-0.34%
Pmtree::override_range	232.0±4.53µs	230.9±4.39µs	-0.47%
Pmtree::set	55.6±0.92µs	54.7±4.99µs	-1.62%
Pmtree:delete	55.6±0.49µs	54.4±1.42µs	-2.16%

[github-actions]

Benchmark for eea865e

Click to view benchmark

Test	Base	PR	%
FullMerkleTree::compute_root	0.0±0.00ns	0.0±0.00ns	NaN%
FullMerkleTree::delete	877.3±14.29ns	875.7±9.65ns	-0.18%
FullMerkleTree::get	3.4±0.14ns	3.4±0.05ns	0.00%
FullMerkleTree::override_range	2.2±0.03µs	2.2±0.04µs	0.00%
FullMerkleTree::set	880.3±17.82ns	877.0±6.06ns	-0.37%
OptimalMerkleTree::compute_root	1035.2±9.65ns	1062.1±14.10ns	+2.60%
OptimalMerkleTree::delete	1038.3±6.81ns	1067.6±15.44ns	+2.82%
OptimalMerkleTree::get	22.9±0.20ns	23.0±0.15ns	+0.44%
OptimalMerkleTree::override_range	5.3±0.08µs	5.4±0.15µs	+1.89%
OptimalMerkleTree::set	1035.1±15.03ns	1064.0±46.51ns	+2.79%