improve @formItemMarginBottom

components/form/style/index.less

@@ -44,13 +44,22 @@
   }
 }
 
-.explainAndExtraDistance(@num) when (@num>=0) {
+.explainAndExtraDistance(@num, @formItemMarginBottom)
+  when
+  (@num >= 0 and @formItemMarginBottom > 0) {
   padding-top: floor(@num);
 }
-.explainAndExtraDistance(@num) when (@num<0) {
+
+.explainAndExtraDistance(@num, @formItemMarginBottom)
+  when
+  (@num < 0 and @formItemMarginBottom > 0) {
   margin-top: floor(@num);
 }
 
+.explainAndExtraDistance(@num, @formItemMarginBottom) when (@formItemMarginBottom <= 0) {
+  display: none;
+}
+
 // ================================================================
 // =                             Item                             =
 // ================================================================
@@ -155,7 +164,7 @@
   &-extra {
     clear: both;
     min-height: @form-item-margin-bottom;
-    .explainAndExtraDistance((@form-item-margin-bottom - @form-font-height) / 2);
+    .explainAndExtraDistance((@form-item-margin-bottom - @form-font-height) / 2, @form-item-margin-bottom);
     color: @text-color-secondary;
     font-size: @font-size-base;
     line-height: @line-height-base;