changed null check in JSONPathMatcher and JmesPathMatcher to ensure that the body is not an empty string (the json parse would throw an exception at this point) (#1278)

src/WireMock.Net/Matchers/JSONPathMatcher.cs

@@ -67,7 +67,7 @@ public class JsonPathMatcher : IStringMatcher, IObjectMatcher
         var score = MatchScores.Mismatch;
         Exception? exception = null;
 
-        if (input != null)
+        if (!string.IsNullOrWhiteSpace(input))
         {
             try
             {

src/WireMock.Net/Matchers/JmesPathMatcher.cs

@@ -74,7 +74,7 @@ public class JmesPathMatcher : IStringMatcher, IObjectMatcher
         var score = MatchScores.Mismatch;
         Exception? exception = null;
 
-        if (input != null)
+        if (!string.IsNullOrWhiteSpace(input))
         {
             try
             {