fix: handle non-existant redis node gracefully

[gurgunday]

A bug introduced in #5 causes iovalkey to throw asynchronously from time to time and crash the instance

Before this change, if the affected methods could not find a node, they would return undefined; however, after the change, they immediately try to access the redis property even if the node doesn't exist at that time, which might happen for a variety of reasons

This is highly problematic as iovalkey runs timeout-based code periodically, and by their nature, it's very hard to catch them

Here's an example of the exception that will crash the instance:

TypeError: Cannot read properties of undefined (reading 'redis')
    at ConnectionPool.getSampleInstance (/node_modules/iovalkey/built/cluster/ConnectionPool.js:31:50)
    at tryConnection (/node_modules/iovalkey/built/cluster/index.js:455:56)
    at DelayQueue.execute (/node_modules/iovalkey/built/cluster/DelayQueue.js:49:26)
    at /node_modules/iovalkey/built/cluster/DelayQueue.js:32:26
    at wrapper (/node_modules/iovalkey/built/cluster/index.js:301:17)
    at tryNode (/node_modules/iovalkey/built/cluster/index.js:309:24)
    at /node_modules/iovalkey/built/cluster/index.js:325:21
    at /node_modules/iovalkey/built/cluster/index.js:666:24
    at run (/node_modules/iovalkey/built/utils/index.js:116:22)
    at tryCatcher (/node_modules/standard-as-callback/built/utils.js:12:23)

[gurgunday]

cc @mcollina

[mcollina]

lgtm

[mcollina]

lgtm