Fix nullability of getFullHeightForCaret (#145554)

Fixes https://github.com/flutter/flutter/issues/145507.

Looks like this was accidentally migrated to nullable all the way back when we switched to NNBD.

packages/flutter/lib/src/painting/text_painter.dart

@@ -1352,7 +1352,7 @@ class TextPainter {
   /// {@endtemplate}
   ///
   /// Valid only after [layout] has been called.
-  double? getFullHeightForCaret(TextPosition position, Rect caretPrototype) {
+  double getFullHeightForCaret(TextPosition position, Rect caretPrototype) {
     final TextBox textBox = _getOrCreateLayoutTemplate().getBoxesForRange(0, 1, boxHeightStyle: ui.BoxHeightStyle.strut).single;
     return textBox.toRect().height;
   }

packages/flutter/lib/src/rendering/editable.dart

@@ -1798,7 +1798,7 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin,
     switch (defaultTargetPlatform) {
       case TargetPlatform.iOS:
       case TargetPlatform.macOS:
-        final double fullHeight = _textPainter.getFullHeightForCaret(caretPosition, caretPrototype) ?? _textPainter.preferredLineHeight;
+        final double fullHeight = _textPainter.getFullHeightForCaret(caretPosition, caretPrototype);
         final double heightDiff = fullHeight - caretRect.height;
         // Center the caret vertically along the text.
         caretRect = Rect.fromLTWH(

packages/flutter/lib/src/rendering/paragraph.dart

@@ -649,7 +649,7 @@ class RenderParagraph extends RenderBox with ContainerRenderObjectMixin<RenderBo
   }
 
   Offset _getOffsetForPosition(TextPosition position) {
-    return getOffsetForCaret(position, Rect.zero) + Offset(0, getFullHeightForCaret(position) ?? 0.0);
+    return getOffsetForCaret(position, Rect.zero) + Offset(0, getFullHeightForCaret(position));
   }
 
   @override
@@ -952,7 +952,7 @@ class RenderParagraph extends RenderBox with ContainerRenderObjectMixin<RenderBo
   /// {@macro flutter.painting.textPainter.getFullHeightForCaret}
   ///
   /// Valid only after [layout].
-  double? getFullHeightForCaret(TextPosition position) {
+  double getFullHeightForCaret(TextPosition position) {
     assert(!debugNeedsLayout);
     _layoutTextWithConstraints(constraints);
     return _textPainter.getFullHeightForCaret(position, Rect.zero);

packages/flutter/test/painting/text_painter_test.dart

@@ -380,7 +380,7 @@ void main() {
       final double caretHeight = painter.getFullHeightForCaret(
         const ui.TextPosition(offset: 0),
         ui.Rect.zero,
-      )!;
+      );
       expect(caretHeight, 50.0);
       painter.dispose();
     }, skip: isBrowser && !isCanvasKit); // https://github.com/flutter/flutter/issues/56308

packages/flutter/test/rendering/paragraph_test.dart

@@ -121,7 +121,7 @@ void main() {
     );
     layout(paragraph);
 
-    final double height5 = paragraph.getFullHeightForCaret(const TextPosition(offset: 5))!;
+    final double height5 = paragraph.getFullHeightForCaret(const TextPosition(offset: 5));
     expect(height5, equals(10.0));
   });