Prevent empty attribute values from being appended (#62)

When adding an attribute that should be appended to any existing one, first check if either
the existing or new attribute contain an empty value, and if so, don't perform the append.
Otherwise, we'll end up with extra whitespace within attribute values. Also, slight cleanup
of the attribute handling logic within `ElementRenderingBuffer`, to make the conditionals
a bit more clear.

Sources/Plot/API/Attribute.swift

@@ -64,7 +64,7 @@ extension Attribute: NodeConvertible {
 
 extension Attribute: AnyAttribute {
     func render() -> String {
-        guard let value = value, !value.isEmpty else {
+        guard let value = nonEmptyValue else {
             return ignoreIfValueIsEmpty ? "" : name
         }
 

Sources/Plot/Internal/AnyAttribute.swift

@@ -11,3 +11,9 @@ internal protocol AnyAttribute {
 
     func render() -> String
 }
+
+extension AnyAttribute {
+    var nonEmptyValue: String? {
+        value?.isEmpty == false ? value : nil
+    }
+}

Sources/Plot/Internal/ElementRenderingBuffer.swift

@@ -20,12 +20,14 @@ internal final class ElementRenderingBuffer {
 
     func add(_ attribute: AnyAttribute) {
         if let existingIndex = attributeIndexes[attribute.name] {
-            if !attribute.replaceExisting,
-               let existingValue = attributes[existingIndex].value,
-               let newValue = attribute.value {
-                attributes[existingIndex].value = existingValue + " " + newValue
-            } else {
+            if attribute.replaceExisting {
                 attributes[existingIndex].value = attribute.value
+            } else if let newValue = attribute.nonEmptyValue {
+                if let existingValue = attributes[existingIndex].nonEmptyValue {
+                    attributes[existingIndex].value = existingValue + " " + newValue
+                } else {
+                    attributes[existingIndex].value = newValue
+                }
             }
         } else {
             attributeIndexes[attribute.name] = attributes.count

Tests/PlotTests/HTMLComponentTests.swift

@@ -88,6 +88,17 @@ final class HTMLComponentTests: XCTestCase {
         XCTAssertEqual(html, #"<p class="one two three">Hello</p>"#)
     }
 
+    func testNotAppendingEmptyClassNames() {
+        let html = Paragraph("Hello")
+            .class("")
+            .class("one")
+            .class("")
+            .class("two")
+            .render()
+
+        XCTAssertEqual(html, #"<p class="one two">Hello</p>"#)
+    }
+
     func testReplacingClass() {
         let html = Paragraph("Hello")
             .class("one")