sc-finality-grandpa: Warp proof generation can not expect justifications (#13249)

When a node is running with `--blocks-pruning` it will also prunes justifications. So, the warp
proof generation can not use `expect` for unwrapping the justification.

substrate/client/cli/src/params/pruning_params.rs

@@ -49,6 +49,7 @@ pub struct PruningParams {
 	/// [default: 256]
 	#[arg(alias = "pruning", long, value_name = "PRUNING_MODE")]
 	pub state_pruning: Option<DatabasePruningMode>,
+
 	/// Specify the blocks pruning mode.
 	///
 	/// This mode specifies when the block's body (including justifications)