All I know is what the Franchise Tax Board tells me.  

"Some key features of a partnership are:

• Partners decide the ownership structure and distribution of profit and losses
• Partnerships file Partnership Return of Income (Form 565)
• Each partners’ income is reported on their individual (Schedule K-1 Form 565)
• Each partner is responsible for paying taxes on their respective tax return."

https://www.ftb.ca.gov/file/business/types/partnerships.html

It doesn't make any difference if the business has no customers in California.  Where do they have offices, warehouses, factories, employees?  And even if those are in China, if  the business has chosen California as its residence, it has also chosen to pay California tax on its profits. 

"We have no customers in California" sounds like someone's feeble excuse for not wanting to pay taxes there.  It won't work.