For the indicator `bt.talib.CORREL`

, if I pass in two indicators with missing values in the middle, no correlation will be calculated after the first missing value, even the values are there afterward, as a demonstration(a and b are both line object, i only show them as array here):

```
import numpy as np
a = np.array([1,1,1,1 ,1,1,1,1,1]) #space on 4th element is for readability
b = np.array([1,1,1,np.nan,1,1,1,1,1])
print(bt.talib.CORREL(a,b,timepriod=3))
>> [nan,nan,1,nan,nan,nan,nan,nan,nan]
```

The intended result should be:

```
[nan,nan,1,nan,nan,nan,1,1,1]
```

In my actual case, both a and b are about 300 days long, and the missing values is about 30 days long in the middle of one data, and I tried to calculate correlation for a 6 days window, not sure if this has to do with issue. All the value before missing ones are correct.