(fix): fix incorrect isEmpty = true during strokeBegin

backport of v0.2.4 addition from 22d8019

- strokeBegin called reset(), which set isEmpty = true , which is a bug
  - moved isEmpty = true logic outside of reset() as the only place it
    actually needs to be set to true is in clear()

Author: agilgur5

src/index.js

@@ -51,6 +51,7 @@ export default class SignatureCanvas extends Component {
     ctx.clearRect(0, 0, canvas.width, canvas.height)
     ctx.fillRect(0, 0, canvas.width, canvas.height)
     this._reset()
+    this._isEmpty = true
   }
 
   fromDataURL = (dataURL) => {
@@ -110,7 +111,6 @@ export default class SignatureCanvas extends Component {
     this.points = [];
     this._lastVelocity = 0;
     this._lastWidth = (this.props.minWidth + this.props.maxWidth) / 2
-    this._isEmpty = true;
     this._ctx.fillStyle = this.props.penColor
   }