Why do some linux header files define a function to return 0 after the declaration?

I am looking at the Linux 4.14 kernel's include/linux/clk.h file, and have noticed that some of the functions are declared, and then later defined to return 0 or NULL.

For example:

struct clk *clk_get(struct device *dev, const char *id);...static inline struct clk *clk_get(struct device *dev, const char *id){        return NULL;}

What is the purpose of doing this? I see multiple C source files that fully define this function and still include linux/clk.h.