ArrayToStringConverter: support {null} case

Avoid throwing an NPE if given a String[] containing a null value.

Closes #444

Author: hinerm

src/main/java/org/scijava/convert/ArrayToStringConverter.java

@@ -93,16 +93,22 @@ private String[] preprocessStrings(final Object src) {
 	}
 
 	private String preprocessString(final Object o) {
-		String s = o.toString();
-		s = s.replace("\\", "\\\\");
-		s = s.replace("\"", "\\\"");
+		String s;
+		if (o == null) {
+			return null;
+		} else {
+			s = o.toString();
+			s = s.replace("\\", "\\\\");
+			s = s.replace("\"", "\\\"");
+		}
 		return "\"" + s + "\"";
 	}
 
 	private String[] preprocessCharacters(Object src) {
 		String[] processed = new String[Array.getLength(src)];
 		for (int i = 0; i < processed.length; i++) {
-			processed[i] = Array.get(src, i).toString();
+			Object value = Array.get(src, i);
+			processed[i] = value == null ? null : value.toString();
 		}
 		return preprocessStrings(processed);
 	}

src/test/java/org/scijava/convert/ArrayToStringConverterTest.java

@@ -156,4 +156,20 @@ public void testCyclicConversion() {
 		Assert.assertArrayEquals(expected, actual);
 	}
 
+	@Test
+	public void testNullConversion() {
+		String[] s1 = {null};
+		String[] s2 = {"null"};
+		String[] expected = new String[] {null};
+		// Do the first conversion
+		ArrayToStringConverter c1 = new ArrayToStringConverter();
+		context.inject(c1);
+		String converted = c1.convert(expected, String.class);
+		// Try to convert back
+		StringToArrayConverter c2 = new StringToArrayConverter();
+		context.inject(c2);
+		String[] actual = c2.convert(converted, String[].class);
+		// NB: we cannot recreate the original {null} state
+		Assert.assertNull(actual);
+	}
 }