Avoid an error if either the issue start_date or the due_date are nil.

This situation can happen on existing data which has a set due_date
but no start_date. The code would then error out with

```
TypeError: expected numeric or date in
[RUBY_ROOT]/lib/ruby/1.8/date.rb:1252:in minus_without_duration
[GEM_ROOT]/gems/activesupport-2.3.15/lib/active_support/core_ext/date/calculations.rb:88:in -
[PROJECT_ROOT]/vendor/plugins/redmine_better_gantt_chart/lib/redmine_better_gantt_chart/issue_dependency_patch.rb:154:in cache_change
[PROJECT_ROOT]/vendor/plugins/redmine_better_gantt_chart/lib/redmine_better_gantt_chart/issue_dependency_patch.rb:69:in reschedule_dependent_issue
[PROJECT_ROOT]/vendor/plugins/redmine_better_gantt_chart/lib/redmine_better_gantt_chart/issue_dependency_patch.rb:87:in process_child_issues
[GEM_ROOT]/gems/activerecord-2.3.15/lib/active_record/named_scope.rb:114:in each
[GEM_ROOT]/gems/activerecord-2.3.15/lib/active_record/named_scope.rb:114:in __send__
[GEM_ROOT]/gems/activerecord-2.3.15/lib/active_record/named_scope.rb:114:in each
[PROJECT_ROOT]/vendor/plugins/redmine_better_gantt_chart/lib/redmine_better_gantt_chart/issue_dependency_patch.rb:78:in process_child_issues
[PROJECT_ROOT]/vendor/plugins/redmine_better_gantt_chart/lib/redmine_better_gantt_chart/issue_dependency_patch.rb:70:in reschedule_dependent_issue
[PROJECT_ROOT]/vendor/plugins/redmine_better_gantt_chart/lib/redmine_better_gantt_chart/issue_dependency_patch.rb:24:in reschedule_following_issues
[PROJECT_ROOT]/vendor/plugins/redmine_better_gantt_chart/lib/redmine_better_gantt_chart/issue_dependency_patch.rb:33:in cache_and_apply_changes
[PROJECT_ROOT]/vendor/plugins/redmine_better_gantt_chart/lib/redmine_better_gantt_chart/issue_dependency_patch.rb:23:in reschedule_following_issues