Don't rely on bignumber.js in @celo/base (#8923)

### Description

There was this small exception to the objective of `@celo/base` not having any dependencies

packages/sdk/base/src/address.ts

@@ -1,5 +1,3 @@
-import BigNumber from 'bignumber.js'
-
 const HEX_REGEX = /^0x[0-9A-F]*$/i
 
 export type Address = string
@@ -8,7 +6,7 @@ export const eqAddress = (a: Address, b: Address) => normalizeAddress(a) === nor
 
 export const normalizeAddress = (a: Address) => trimLeading0x(a).toLowerCase()
 
-export const isNullAddress = (a: Address) => new BigNumber(normalizeAddress(a)).isZero()
+export const isNullAddress = (a: Address) => normalizeAddress(a) === NULL_ADDRESS
 
 export const normalizeAddressWith0x = (a: Address) => ensureLeading0x(a).toLowerCase()