is there a calculation for no load current on BLDC motor?
No load current is zero amperes.

KV 2100 = 2100 RPM/V = 35 R/Vs = 220 rad/Vs

So, Ke,Kt = 1/220 = 0.00455 [Vs/rad] = 0.00455 [Nm/A] 

$$U=K_e\cdot\omega + R\cdot I $$

EDIT:

\$T=K_e\cdot I \$   or  \$I=\dfrac{T}{K_e}\$ so if T=0 then current is zero. Else you would have to know the no load torque, which could be the ball bearing friction and air drag at known speed.

The derived Ke from KV that was actually measured in laboratory consists of a test setup. This is a second motor coupled to the shaft and spin as much as the current is zero - no load current. In such situation the applied voltage equals the back emf voltage \$ U=K_e\cdot\omega\$, knowing \$U\$ and \$\omega\$ is it possible to estimate \$K_e\$ or KV in your case. So the motor is spinning at 35,500 RPM at 15V, but not by himself, rather by the help of the coupled motor from the setup.