fix JDK 15 build (#112)

Author: sullis

.github/workflows/build.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: [ '8', '11', '14', '15-ea' ]
+        java: [ '8', '11', '14', '15' ]
         architecture: [ 'x64' ]
     name: Build with JDK ${{ matrix.java }} on ${{ matrix.architecture }}
     steps:

src/test/java/org/assertj/vavr/api/MapAssert_containsAllEntriesOf_Test.java

@@ -60,8 +60,7 @@ void should_fail_if_entries_parameter_is_null() {
         assertThatThrownBy(
                 () -> assertThat(HashMap.<String, String>empty()).containsAllEntriesOf(null)
         )
-                .isInstanceOf(NullPointerException.class)
-                .hasMessage(null);
+                .isInstanceOf(NullPointerException.class);
     }
 
     @Test

src/test/java/org/assertj/vavr/api/MultimapAssert_containsAllEntriesOf_Test.java

@@ -58,8 +58,7 @@ void should_fail_if_entries_parameter_is_null() {
         assertThatThrownBy(
                 () -> assertThat(HashMultimap.withSeq().<String, String>empty()).containsAllEntriesOf(null)
         )
-                .isInstanceOf(NullPointerException.class)
-                .hasMessage(null);
+                .isInstanceOf(NullPointerException.class);
     }
 
     @Test