Add more specs and handle errors related to time parsing.

app/services/slash_commands/interpret_service.rb

@@ -254,7 +254,7 @@ module SlashCommands
       current_user.can?(:"admin_#{issuable.to_ability_name}", project)
     end
     command :estimate do |raw_duration|
-      time_spent = ChronicDuration.parse(raw_duration, default_unit: 'hours')
+      time_spent = ChronicDuration.parse(raw_duration, default_unit: 'hours') rescue nil
 
       if time_spent
         @updates[:time_estimate] = time_spent
@@ -268,7 +268,7 @@ module SlashCommands
     end
     command :spend do |raw_duration|
       reduce_time = raw_duration.sub!(/\A-/, '')
-      time_spent = ChronicDuration.parse(raw_duration, default_unit: 'hours')
+      time_spent = ChronicDuration.parse(raw_duration, default_unit: 'hours') rescue nil
 
       if time_spent
         @updates[:spend_time] = reduce_time ? (time_spent * -1) : time_spent

spec/services/slash_commands/interpret_service_spec.rb

@@ -472,11 +472,31 @@ describe SlashCommands::InterpretService, services: true do
       let(:issuable) { issue }
     end
 
+    it_behaves_like 'empty command' do
+      let(:content) { '/estimate' }
+      let(:issuable) { issue }
+    end
+
+    it_behaves_like 'empty command' do
+      let(:content) { '/estimate abc' }
+      let(:issuable) { issue }
+    end
+
     it_behaves_like 'spend command' do
       let(:content) { '/spend 1h' }
       let(:issuable) { issue }
     end
 
+    it_behaves_like 'empty command' do
+      let(:content) { '/spend' }
+      let(:issuable) { issue }
+    end
+
+    it_behaves_like 'empty command' do
+      let(:content) { '/spend abc' }
+      let(:issuable) { issue }
+    end
+
     context 'when current_user cannot :admin_issue' do
       let(:visitor) { create(:user) }
       let(:issue) { create(:issue, project: project, author: visitor) }