use undefined not null

packages/mermaid/src/diagrams/sequence/svgDraw.js

@@ -218,7 +218,11 @@ export const drawText = function (elem, textData) {
     }
   }
   for (let [i, line] of lines.entries()) {
-    if (textData.textMargin !== undefined && textData.textMargin === 0 && _textFontSize !== null) {
+    if (
+      textData.textMargin !== undefined &&
+      textData.textMargin === 0 &&
+      _textFontSize !== undefined
+    ) {
       dy = i * _textFontSize;
     }
 
@@ -234,7 +238,7 @@ export const drawText = function (elem, textData) {
     if (textData.fontFamily !== undefined) {
       textElem.style('font-family', textData.fontFamily);
     }
-    if (_textFontSizePx !== null) {
+    if (_textFontSizePx !== undefined) {
       textElem.style('font-size', _textFontSizePx);
     }
     if (textData.fontWeight !== undefined) {

packages/mermaid/src/utils.spec.js

@@ -421,10 +421,10 @@ describe('when parsing font sizes', function () {
   });
 
   it('handles undefined input', function () {
-    expect(utils.parseFontSize(undefined)).toEqual([null, null]);
+    expect(utils.parseFontSize(undefined)).toEqual([undefined, undefined]);
   });
 
   it('handles unparseable input', function () {
-    expect(utils.parseFontSize({ fontSize: 14 })).toEqual([null, null]);
+    expect(utils.parseFontSize({ fontSize: 14 })).toEqual([undefined, undefined]);
   });
 });

packages/mermaid/src/utils.ts

@@ -962,7 +962,7 @@ export const parseFontSize = (fontSize: string | number | undefined): [number?,
   const fontSizeNumber = parseInt(fontSize, 10);
   if (Number.isNaN(fontSizeNumber)) {
     // if a number value can't be parsed, return null for both values
-    return [null, null];
+    return [undefined, undefined];
   } else if (fontSize === String(fontSizeNumber)) {
     // if a string input doesn't contain any units, assume px units
     return [fontSizeNumber, fontSize + 'px'];