Internal Compiler Error: Parsing Source Maps

Just brought in master. My guess is this was caused by 200824732982ab6032e53de74eee59a49c775bc0

Didn't occur with my last build from about a week ago.

```
java.lang.UnsupportedOperationException: JsonNull
at com.google.gson.JsonElement.getAsString(JsonElement.java:191)
at com.google.debugging.sourcemap.SourceMapObjectParser.getJavaStringArray(SourceMapObjectParser.java:103)
at com.google.debugging.sourcemap.SourceMapObjectParser.parse(SourceMapObjectParser.java:55)
at com.google.debugging.sourcemap.SourceMapConsumerV3.parse(SourceMapConsumerV3.java:69)
at com.google.javascript.jscomp.SourceMapInput.getSourceMap(SourceMapInput.java:58)
at com.google.javascript.jscomp.Compiler.getSourceMapping(Compiler.java:2922)
at com.google.javascript.jscomp.LightweightMessageFormatter.format(LightweightMessageFormatter.java:95)
at com.google.javascript.jscomp.LightweightMessageFormatter.formatWarning(LightweightMessageFormatter.java:79)
at com.google.javascript.jscomp.JSError.format(JSError.java:171)
at com.google.javascript.jscomp.PrintStreamErrorManager.println(PrintStreamErrorManager.java:58)
at com.google.javascript.jscomp.BasicErrorManager.generateReport(BasicErrorManager.java:62)
at com.google.javascript.jscomp.ThreadSafeDelegatingErrorManager.generateReport(ThreadSafeDelegatingErrorManager.java:37)
at com.google.javascript.jscomp.Compiler.generateReport(Compiler.java:717)
at com.google.javascript.jscomp.AbstractCommandLineRunner.performFullCompilation(AbstractCommandLineRunner.java:1239)
at com.google.javascript.jscomp.AbstractCommandLineRunner.doRun(AbstractCommandLineRunner.java:1159)
at com.google.javascript.jscomp.AbstractCommandLineRunner.run(AbstractCommandLineRunner.java:516)
at com.google.javascript.jscomp.CommandLineRunner.main(CommandLineRunner.java:2050)
```