common : make string_split<T> throw on invalid values

[angt]

Overview

Before:

$ llama-bench -hf unsloth/Qwen3.5-0.8B-GGUF  -nkvo 1,,0 -p 0
| model                          |       size |     params | backend    | threads |          nkvo |            test |                  t/s |
| ------------------------------ | ---------: | ---------: | ---------- | ------: | ------------: | --------------: | -------------------: |
| qwen35 0.8B Q4_K - Medium      | 497.39 MiB |   752.39 M | CPU        |      16 |             1 |           tg128 |         86.32 ± 0.42 |
| qwen35 0.8B Q4_K - Medium      | 497.39 MiB |   752.39 M | CPU        |      16 |             1 |           tg128 |         86.05 ± 0.09 |
| qwen35 0.8B Q4_K - Medium      | 497.39 MiB |   752.39 M | CPU        |      16 |             0 |           tg128 |         87.10 ± 0.21 |

After:

$ llama-bench -hf unsloth/Qwen3.5-0.8B-GGUF  -nkvo 1,,0 -p 0
error: invalid value: ""
error: invalid parameter for argument: -nkvo
...

Additional information

Requirements

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