Embedding text files that start with multiple newlines is broken

### Description

I believe this is a limitation of the YAML libraries we are using, so there might not be much we can do about that (this is a very longstanding issue). But we should figure out the exact conditions when things are broken, and provide a reasonable error message.

Example:

```console
❯ cat tmpl.yaml
base: template://alpine
provision:
- mode: data
file: sample.data
path: /tmp/data

❯ echo $'\nA' > sample.data
❯ limactl tmpl copy --embed-all tmpl.yaml -
WARN[0000] `base` is experimental
WARN[0000] `provision[*].file` and `probes[*].file` are experimental
provision:
- mode: data
content: |
A
path: /tmp/data
…

❯ echo $'\n\nA' > sample.data
❯ limactl tmpl copy --embed-all tmpl.yaml -
WARN[0000] `base` is experimental
WARN[0000] `provision[*].file` and `probes[*].file` are experimental
FATA[0000] yaml: line 1: did not find expected key
``