fix: Make build field optional

CHANGELOG.adoc

@@ -5,6 +5,10 @@
 
 == DFX
 
+=== fix: make `+build+` field optional for custom canisters
+
+Prior to 0.11.0, a custom canister's `+build+` field could be left off if `+dfx build+` was never invoked. To aid in deploying prebuilt canisters, this behavior is now formalized; omitting `+build+` is equivalent to `+build: []+`.
+
 === feat: Use `+--locked+` for Rust canisters
 
 `+dfx build+`, in Rust canisters, now uses the `+--locked+` flag when building with Cargo. To offset this, `+dfx new --type rust+` now runs `+cargo update+` on the resulting project.

src/dfx/src/config/dfinity.rs

@@ -107,6 +107,7 @@ pub enum CanisterTypeProperties {
     Custom {
         wasm: PathBuf,
         candid: PathBuf,
+        #[serde(default)]
         build: SerdeVec<String>,
     },
     Motoko,