[Electric-Coin-Company#1420] Synchronizer.proposeShielding crashes wh…

…en there are insufficient funds to shield

- Check for the null value of the pointer when constructing propose shielding

[Electric-Coin-Company#1420] Synchronizer.proposeShielding crashes when there are insufficient funds to shield

- throw an error instead of returning nil value

Sources/ZcashLightClientKit/Rust/ZcashRustBackend.swift

@@ -756,7 +756,11 @@ struct ZcashRustBackend: ZcashRustBackendWelding {
         }
 
         defer { zcashlc_free_boxed_slice(proposal) }
-
+
+        guard proposal.pointee.ptr != nil else {
+            throw ZcashError.rustShieldFunds(lastErrorMessage(fallback: "`proposeShielding` insufficient funds to shield"))
+        }
+
         return try FfiProposal(contiguousBytes: Data(
             bytes: proposal.pointee.ptr,
             count: Int(proposal.pointee.len)