refactor: allow for customSheetName to be an empty string

While you could do this before, if you had a class `a` and you wanted to
keep it simple and not have anything for your `customSheetName` you'd
end up with `-a` which was a bit annoying. This small change allows
users to still use this the way the were before, but now if they are
using an empty string as their `customSheetName`, they'll just get `a`
instead of `-a`.

scalatags/src/scalatags/stylesheet/StyleSheet.scala

@@ -21,7 +21,7 @@ abstract class CascadingStyleSheet(implicit sourceName: sourcecode.FullName) ext
 abstract class StyleSheet(implicit sourceName: sourcecode.FullName){
   /**
    * The name of this CSS stylesheet. Defaults to the name of the trait,
-   * but you can override
+   * but you can override.
    */
   def customSheetName: Option[String] = None
 
@@ -30,7 +30,12 @@ abstract class StyleSheet(implicit sourceName: sourcecode.FullName){
    * any applicable pseudo-selectors into the name of the CSS class.
    */
   protected[this] def nameFor(memberName: String, pseudoSelectors: String) = {
-    customSheetName.getOrElse(defaultSheetName.replace(".", "-")) + "-" + memberName + pseudoSelectors
+    val baseSuffix = memberName + pseudoSelectors
+    customSheetName match {
+      case Some(value) if value.isEmpty() => baseSuffix
+      case Some(value) => value + "-" + baseSuffix
+      case None => defaultSheetName.replace(".", "-") + "-" + baseSuffix
+    }
   }
 
   /**

scalatags/test/src/scalatags/generic/StyleSheetTests.scala

@@ -83,6 +83,16 @@ abstract class StyleSheetTests[Builder, Output <: FragT, FragT]
 
   }
 
+  object CustomEmpty extends CascadingStyleSheet{
+    initStyleSheet()
+
+    override def customSheetName = Some("")
+    val x = cls(
+      backgroundColor := "red",
+      height := 125
+    )
+  }
+
 
   def check(txt: String, expected: String) = {
     // augmentString = work around scala/bug#11125 on JDK 11
@@ -168,6 +178,17 @@ abstract class StyleSheetTests[Builder, Output <: FragT, FragT]
          """
       )
     }
+    test("customEmpty"){
+      check(
+        CustomEmpty.styleSheetText,
+        s"""
+          .x{
+            background-color: red;
+            height: 125px;
+          }
+         """
+      )
+    }
     test("failure"){
       test("noDirectInstantiation"){
   // This doesn't seem to work, even though that snippet does indeed