Fix two CombinationResultsConverter edge cases

[SimonCropp]

Unclosed object for empty results. Write calls WriteStartObject(), then returns early when there are no items without calling WriteEndObject(). The writer auto-closes at the end of the document, so a top level empty result still renders as {} and looks fine — but anything written after the results gets swallowed into them. On main:

{
  results: {
    after: TheValue     <- sibling property, now nested inside the results
  }
}

Only reachable by constructing CombinationResults directly, since CombinationRunner rejects empty lists.

Flattened messages lost the joining space. FlattenMessage appended ". " only when a line did not already end in a period, so a line that did was run straight into the next one. The two line net48 ArgumentNullException rendered as:

ArgumentException: Value cannot be null.Parameter name: p.

The period and the separating space are now appended independently.

Tests in Verify.Tests/CombinationTests: EmptyResults and EmptyResultsNested for the first (the nested one is what actually fails on main — the flat one passes either way, which is why the bug is easy to miss), and MultiLineArgumentException for the second, using an explicit two line message so it does not depend on how a given framework words ArgumentNullException.

Empty lines in a message still contribute a . — unchanged, and out of scope here.

Verify.Tests (1302) and StaticSettingsTests pass. docs/combinations.md regenerates because it embeds the converter source.