fix(NumberPicker): repair numberPicker log string, close alibaba-fusion#3768 (alibaba-fusion#3769)

Author: weinianyang

src/number-picker/number-picker.jsx

@@ -427,12 +427,16 @@ class NumberPicker extends React.Component {
             return onDisabled(e);
         }
 
-        const value = this.state.value;
+        let value = this.state.value;
         // 受控下，可能强制回填非法值
         if (isNaN(value)) {
             return;
         }
 
+        if (value === '' && !this.props.stringMode) {
+            value = 0;
+        }
+
         let val = this[`${type}Step`](value);
         val = this.correctBoundary(val);
         // 受控下，显示的值应为受控value