[SuperEditor] Fix crash with selected color strategy in an empty paragraph (Resolves #2253)

[angelosilvestre]

[SuperEditor] Fix crash with selected color strategy in an empty paragraph (Resolves #2253)

When the editor has a selectedTextColorStrategy configured, triple tapping on an empty paragraph crashes the editor with the following exception:

[ERROR:flutter/runtime/dart_vm_initializer.cc(40)] Unhandled Exception: 'package:attributed_text/src/span_range.dart': Failed assertion: line 20 pos 16: 'end >= -1': is not true.
#0      _AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:63:4)
#1      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:45:5)
#2      new SpanRange (package:attributed_text/src/span_range.dart:20:16)
#3      SingleColumnLayoutSelectionStyler._applySelection (package:super_editor/src/default_editor/layout_single_column/_styler_user_selection.dart:147:17)
#4      SingleColumnLayoutSelectionStyler.style (package:super_editor/src/default_editor/layout_single_column/_styler_user_selection.dart:79:11)
#5      SingleColumnLayoutPresenter._createNewViewModel (package:super_editor/src/default_editor/layout_single_column/_presenter.dart:189:35)
#6      SingleColumnLayoutPresenter.updateViewModel (package:super_editor/src/default_editor/layout_single_column/_presenter.dart:148:18)
#7      _SingleColumnDocumentLayoutState._onPresenterMarkedDirty (package:super_editor/src/default_editor/layout_single_column/_layout.dart:115:22)
#8      SingleColumnLayoutPresenterChangeListener.onPresenterMarkedDirty (package:super_editor/src/default_editor/layout_single_column/_presenter.dart:338:30)
#9      SingleColumnLayoutPresenter._assemblePipeline.<anonymous closure> (package:super_editor/src/default_editor/layout_single_column/_presenter.dart:125:22)
#10     SingleColumnLayoutStylePhase.markDirty (package:super_editor/src/default_editor/layout_single_column/_presenter.dart:407:21)
#11     ChangeNotifier.notifyListeners (package:flutter/src/foundation/change_notifier.dart:439:24)
#12     ValueNotifier.value= (package:flutter/src/foundation/change_notifier.dart:564:5)
#13     PausableValueNotifier.resumeNotifications (package:super_editor/src/infrastructure/pausable_value_notifier.dart:49:11)
#14     MutableDocumentComposer.onTransactionEnd (package:super_editor/src/core/document_composer.dart:155:24)
#15     Editor._onTransactionEnd (package:super_editor/src/core/editor.dart:331:16)
#16     Editor.endTransaction (package:super_editor/src/core/editor.dart:231:5)
#17     Editor.execute (package:super_editor/src/core/editor.dart:283:7)
#18     _DocumentMouseInteractorState._selectParagraphAt (package:super_editor/src/default_editor/document_gestures_mouse.dart:499:21)
#19     _DocumentMouseInteractorState._onTripleTapDown (package:super_editor/src/default_editor/document_gestures_mouse.dart:479:34)
#20     TapSequenceGestureRecognizer.addAllowedPointer.<anonymous closure> (package:super_editor/src/infrastructure/multi_tap_gesture.dart:105:71)
#21     GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:357:24)
#22     TapSequenceGestureRecognizer.addAllowedPointer (package:super_editor/src/infrastructure/multi_tap_gesture.dart:105:9)
#23     GestureRecognizer.addPointer (package:flutter/src/gestures/recognizer.dart:254:7)
#24     RawGestureDetectorState._handlePointerDown (package:flutter/src/widgets/gesture_detector.dart:1550:18)
#25     RenderPointerListener.handleEvent (package:flutter/src/rendering/proxy_box.dart:3095:44)
#26     GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:499:22)
#27     RendererBinding.dispatchEvent (package:flutter/src/rendering/binding.dart:460:11)
#28     GestureBinding._handlePointerEventImmediately (package:flutter/src/gestures/binding.dart:437:7)
#29     GestureBinding.handlePointerEvent (package:flutter/src/gestures/binding.dart:394:5)
#30     GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:341:7)
#31     GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:308:9)
#32     _invoke1 (dart:ui/hooks.dart:332:13)
#33     PlatformDispatcher._dispatchPointerDataPacket (dart:ui/platform_dispatcher.dart:451:7)
#34     _dispatchPointerDataPacket (dart:ui/hooks.dart:267:31)

This PR fixes this issue by checking if the selection is not collapsed before trying to apply the attributions.