On and off 1099-NEC Schedule C Over the Years

If a taxpayer receives a 1099-NEC for their work along with their usual annual W-2 income but it is not consistent and some years they have and some years they do not. Question is how are the years without a 1099-NEC reported? This may be non-inclusive of everything but for example expenses, items bought for the work that was paid for, mileage and vehicle usage for the work paid for, onward and etc. Do you not depreciate an item bought or used? Do you just leave out the Schedule C all together and report nothing if no 1099-NEC received for that tax year? Do you include expenses even if its a negative due to no 1099-NEC income or is that an ask for an audit if over 2 to 3 to 4 years of no 1099-NEC issued? Please help with a detailed response to this scenario with examples and references. Thanks.