JavaKit: map NULL jstring to the empty string

There's an impedance mismatch here between the JNIType (which is optional) and
the Swift type. Assuming this is intentional than init(fromJNI:in) should check
for a NULL value and map it to the empty string.

Fixes: #155

Author: lhoward

Sources/JavaKit/BridgedValues/JavaValue+String.swift

@@ -24,6 +24,10 @@ extension String: JavaValue {
   }
 
   public init(fromJNI value: JNIType, in environment: JNIEnvironment) {
+    guard let value else {
+      self.init()
+      return
+    }
     let cString = environment.interface.GetStringUTFChars(environment, value, nil)!
     defer { environment.interface.ReleaseStringUTFChars(environment, value, cString) }
     self = String(cString: cString)

Tests/JavaKitTests/BasicRuntimeTests.swift

@@ -76,6 +76,12 @@ class BasicRuntimeTests: XCTestCase {
       XCTAssertEqual(String(describing: error), "org/swift/javakit/Nonexistent")
     }
   }
+
+  func testNullJavaStringConversion() throws {
+    let environment = try jvm.environment()
+    let nullString = String(fromJNI: nil, in: environment)
+    XCTAssertEqual(nullString, "")
+  }
 }
 
 @JavaClass("org.swift.javakit.Nonexistent")