refactor: use unwrap_or_default for optional metadata maps

[immanuwell]

5 methods in client/mod.rs all have the same verbose pattern for handling optional metadata:

let mut mdata = HashMap::<String, String>::new();
if let Some(m) = metadata {
    mdata = m;
}

This is just a roundabout way of writing metadata.unwrap_or_default(). Swaps all 5 instances for that. No behavior change - HashMap::default() is identical to HashMap::new().

Also removes the unnecessary mut on the bindings where the map isn't modified after init (4 out of 5 cases).

To reproduce the pattern before the fix:

grep -n "let mut mdata = HashMap" dapr/src/client/mod.rs

[Copilot]

Pull request overview

This PR refactors repeated optional-metadata handling in dapr/src/client/mod.rs by replacing a verbose HashMap::new() + if let Some(...) pattern with the idiomatic metadata.unwrap_or_default(), aligning with similar existing usage in the same module.

Changes:

• Replaces five occurrences of manual Option<HashMap<...>> unwrapping with unwrap_or_default().
• Removes unnecessary mut bindings where the metadata map is not mutated after initialization.
• Keeps mut only where metadata is subsequently modified (e.g., adding "Content-Type" in invoke_actor).

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