Don't skip empty lines inside DTrace stracks

[This `if`](https://github.com/jonhoo/inferno/blob/76508029643aba8cadc9b306cdf115469df8bb88/src/collapse/dtrace.rs#L132L134) makes me think that empty lines _within_ a stack (i.e., after a non-empty line and before a count) should _not_ [be skipped](https://github.com/jonhoo/inferno/blob/76508029643aba8cadc9b306cdf115469df8bb88/src/collapse/dtrace.rs#L65L66).

Start in src/collapse/dtrace.rs at the linked conditionals around lines 65-66 and 132-134. Check how an empty line between a non-empty stack line and its count is handled; done means such an empty line is no longer skipped.

Written by the indexing model from the issue text.