forcast generation sometimes fails, and REST API resturns nans

[savelov]

Need to investigate
{
'probability': '0.0',
'timestamp': '2019-07-24T11:40:00Z'
},
{
'probability': '0.0',
'timestamp': '2019-07-24T11:50:00Z'
},
{
'probability': '0.0',
'timestamp': '2019-07-24T12:00:00Z'
},
{
'probability': 'nan',
'timestamp': '2019-07-24T12:10:00Z'
},
{
'probability': 'nan',
'timestamp': '2019-07-24T12:20:00Z'
},
{
'probability': 'nan',
'timestamp': '2019-07-24T12:30:00Z'
},
{
'probability': 'nan',
'timestamp': '2019-07-24T12:40:00Z'
}
]