Fix helm-dash-at-point when no symbol at point

[otsaw]

When point is on a space, blank line, punctuation etc. use a blank string as input instead to nil to avoid "Wrong type argument: stringp, nil" from substring-no-properties.

The problem was probably introduced recently in b7fbf789b83e290885e506e887f6ed9c5b858423.

[kidd]

good catch. Applied!

Thanks!