[Minor] Speedup to_array_of_size for Decimal128

[Dandandan]

Which issue does this PR close?

Rationale for this change

This came up in some profiles (mainly query 1, 16).
Benchmark results:

Benchmark tpch_mem.json
--------------------
┏━━━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━┓
┃ Query        ┃     main ┃ speedup_scalar_to_array_decimal ┃        Change ┃
┡━━━━━━━━━━━━━━╇━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━┩
│ QQuery 1     │ 199.40ms │                        182.91ms │ +1.09x faster │
│ QQuery 2     │  34.24ms │                         33.64ms │     no change │
│ QQuery 3     │  46.25ms │                         45.73ms │     no change │
│ QQuery 4     │  36.88ms │                         37.35ms │     no change │
│ QQuery 5     │  94.34ms │                         94.33ms │     no change │
│ QQuery 6     │  10.33ms │                         10.22ms │     no change │
│ QQuery 7     │ 190.04ms │                        194.06ms │     no change │
│ QQuery 8     │  70.00ms │                         71.05ms │     no change │
│ QQuery 9     │ 134.55ms │                        138.00ms │     no change │
│ QQuery 10    │  91.41ms │                         90.04ms │     no change │
│ QQuery 11    │  40.19ms │                         39.95ms │     no change │
│ QQuery 12    │  67.83ms │                         67.38ms │     no change │
│ QQuery 13    │ 104.36ms │                        103.15ms │     no change │
│ QQuery 14    │  12.28ms │                         11.64ms │ +1.05x faster │
│ QQuery 15    │  12.64ms │                         12.81ms │     no change │
│ QQuery 16    │  40.80ms │                         36.65ms │ +1.11x faster │
│ QQuery 17    │ 111.00ms │                        112.18ms │     no change │
│ QQuery 18    │ 296.72ms │                        292.79ms │     no change │
│ QQuery 19    │  57.91ms │                         58.56ms │     no change │
│ QQuery 20    │  79.58ms │                         78.76ms │     no change │
│ QQuery 21    │ 257.55ms │                        254.48ms │     no change │
│ QQuery 22    │  27.98ms │                         28.76ms │     no change │
└──────────────┴──────────┴─────────────────────────────────┴───────────────┘

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[alamb]

LGTM -- thanks @Dandandan