Improve the handling getBaseStreams, on the various Stream implementations

The way that `getBaseStreams` is currently handled has bothered me from time to time, especially how we're checking if the method exists before calling it.
By adding a dummy `BaseStream.getBaseStreams` method, and having the call-sites simply check the return value, we can improve some of the relevant code.

Note in particular how the `ObjectLoader._walk` method didn't actually check that the data in question is a Stream instance, and instead only checked the `currentNode` (which could be anything) for the existence of a `getBaseStreams` property.

src/core/base_stream.js

@@ -94,6 +94,13 @@ class BaseStream {
   makeSubStream(start, length, dict = null) {
     unreachable("Abstract method `makeSubStream` called");
   }
+
+  /**
+   * @returns {Array | null}
+   */
+  getBaseStreams() {
+    return null;
+  }
 }
 
 export { BaseStream };

src/core/decode_stream.js

@@ -122,10 +122,7 @@ class DecodeStream extends BaseStream {
   }
 
   getBaseStreams() {
-    if (this.str && this.str.getBaseStreams) {
+    return this.str ? this.str.getBaseStreams() : null;
-      return this.str.getBaseStreams();
-    }
-    return [];
   }
 }
 
@@ -159,13 +156,14 @@ class StreamsSequenceStream extends DecodeStream {
   }
 
   getBaseStreams() {
-    const baseStreams = [];
+    const baseStreamsBuf = [];
     for (const stream of this.streams) {
-      if (stream.getBaseStreams) {
+      const baseStreams = stream.getBaseStreams();
-        baseStreams.push(...stream.getBaseStreams());
+      if (baseStreams) {
+        baseStreamsBuf.push(...baseStreams);
       }
     }
-    return baseStreams;
+    return baseStreamsBuf.length > 0 ? baseStreamsBuf : null;
   }
 }
 

src/core/object_loader.js

@@ -108,8 +108,9 @@ class ObjectLoader {
           pendingRequests.push({ begin: ex.begin, end: ex.end });
         }
       }
-      if (currentNode && currentNode.getBaseStreams) {
+      if (isStream(currentNode)) {
         const baseStreams = currentNode.getBaseStreams();
+        if (baseStreams) {
           let foundMissingData = false;
           for (const stream of baseStreams) {
             if (stream.isDataLoaded) {
@@ -122,6 +123,7 @@ class ObjectLoader {
             nodesToRevisit.push(currentNode);
           }
         }
+      }
 
       addChildren(currentNode, nodesToVisit);
     }