prefer warning over erroring out when parsing java manifests (#688)

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

syft/pkg/cataloger/java/parse_java_manifest.go

@@ -46,7 +46,8 @@ func parseJavaManifest(path string, reader io.Reader) (*pkg.JavaManifest, error)
 			// this is a continuation
 
 			if lastKey == "" {
-				return nil, fmt.Errorf("found continuation with no previous key (%s)", line)
+				log.Warnf("java manifest %q: found continuation with no previous key: %q", path, line)
+				continue
 			}
 
 			sections[currentSection()][lastKey] += strings.TrimSpace(line)
@@ -54,21 +55,26 @@ func parseJavaManifest(path string, reader io.Reader) (*pkg.JavaManifest, error)
 			continue
 		}
 
-		if lastKey == "" {
-			// we're entering a new section
-
-			sections = append(sections, make(map[string]string))
-		}
-
 		// this is a new key-value pair
 		idx := strings.Index(line, ":")
 		if idx == -1 {
-			return nil, fmt.Errorf("unable to split java manifest key-value pairs: %q", line)
+			log.Warnf("java manifest %q: unable to split java manifest key-value pairs: %q", path, line)
+			continue
 		}
 
 		key := strings.TrimSpace(line[0:idx])
 		value := strings.TrimSpace(line[idx+1:])
 
+		if key == "" {
+			// don't attempt to add new keys or sections unless there is a non-empty key
+			continue
+		}
+
+		if lastKey == "" {
+			// we're entering a new section
+			sections = append(sections, make(map[string]string))
+		}
+
 		sections[currentSection()][key] = value
 
 		// keep track of key for potential future continuations

syft/pkg/cataloger/java/parse_java_manifest_test.go

@@ -70,6 +70,15 @@ func TestParseJavaManifest(t *testing.T) {
 					"thing-1": {
 						"Built-By": "?",
 					},
+					"thing-2": {
+						"Built-By": "someone!",
+					},
+					"2": {
+						"Other": "things",
+					},
+					"3": {
+						"Last": "item",
+					},
 				},
 			},
 		},

syft/pkg/cataloger/java/test-fixtures/manifest/extra-empty-lines

@@ -5,3 +5,18 @@ Created-By: Apache Maven 3.6.3
 
 Name: thing-1
 Built-By: ?
+
+.
+
+Name: thing-2
+Built-By: someone!
+
+
+ :
+
+Other: things
+junk
+
+:
+
+Last: item