[SDK] Fix: allow 0 values for gasPrice (#5841)

## Problem solved

Allow `0` value for gasPrice, important for gasless chains
Fixes TOOL-2865

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## PR-Codex overview
This PR modifies the conditional check for `gasPrice` in the `fee-data.ts` file to ensure it specifically checks if `gasPrice` is of type `bigint` instead of just checking its truthiness.

### Detailed summary
- Changed the condition from `if (gasPrice)` to `if (typeof gasPrice === "bigint")` in the `fee-data.ts` file.
- This ensures that only values of type `bigint` are considered valid for `gasPrice`.

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packages/thirdweb/src/gas/fee-data.ts

@@ -61,7 +61,7 @@ export async function getGasOverridesForTransaction(
       maxPriorityFeePerGas,
     };
   }
-  if (gasPrice) {
+  if (typeof gasPrice === "bigint") {
     return { gasPrice };
   }